Least Favorite:
I came in to say Cats. Blech. I especially hate that song they keep singing. And in the production I saw all the cats looked the same. Not that, from what I remember, there was a plot to follow.
Favorite:
The Sound of Music is my favorite musical ever.
1776 It’s a musical about the founding fathers, what’s not to like? “Saltpeter Abigail! Pins John!”
My Fair Lady is also great.
I love this topic, too! Unfortunately, most of my exposure to musicals is through cast recordings, since Wisconsin is a long way from New York, and the shows I want to see have a tendency to go through Chicago during finals week.
Shows I Love
– Avenue Q: Maybe it’s because I’m an English major, maybe it’s because it’s full of puppets, or maybe it’s because there’s a song about underwear. It’s probably all three.
– 1776: If I could play Richard Henry Lee (of old Virginia), I would. Unfortunately, I don’t think there’s much call for a 5-foot-2-inch first alto to sing that part.
– Jesus Christ Superstar: I have listened to the cast recording of this for hours on end, and only stopped because I was asked to. This may or may not have something to do with Simon “Hottie” Zealotes (Tony Vincent).
– Wicked: It’s gorgeous, through and through. “Popular” is tremendous fun.
– The Batboy Musical: I have no shame.
Shows I HATE
– South Pacific: It has not aged well at all. I know it’s supposed to be all romantic, but I find the rich French plantation owner guy to be extremely annoying and unattractive as a character. Some enchanted evening, you may find a guy with a funny accent bellowing in your ear.
– Bye-Bye Birdie: Another one that’s very dated, but bizarrely loved by many. With zany stereotypes!
– Jekyll/Hyde: I think that’s what it’s called. It starred David Hasselhoff in the title role. Oh my god, excrutiating. I caught it on HBO one day during high school when I had the flu. It completely boggles me that someone would record it – maybe it was part of an elaborate joke?
Shows that I Like Most of, but Not All
– Les Mis: Some of the songs are absolutely gorgeous, and I love most of it. But truly? I find Cosette incredibly tiresome, and I think she’s pretty much a plot device to bring Valjean and the student revolutionaries together.
– Hair: It’s got a song about sodomy! But the stage version is a mess, so it gets bumped from “love” to only “like.”
Shows I like:
Les Miserables: Saw it on Broadway this summer for the first time and fell in love. Awesome all the way through. Loved the rousing, epic songs and the themes of honor, loyalty, and redemption. Jean Valjean is a superhero!
Rent: Seen it three times, including on Broadway with the original Mark (Anthony Rapp) and Roger (Adam Pascal). Really like the songs and the staging, and even dig the movie adaptation. However, I admit most of the characters are narcissistic and obnoxious, and I totally agree with Sampiro that Benny is one of the more sympathetic characters (being the one who tries to help everyone, even when they won’t have any of it.)
Little Shop of Horrors: Only ever seen the movie, but it’s one of my all-time favorites. I always wanted to play Seymour, and I could do all the songs from memory right now. Yes, I realize he is a villain, even if not THE villain.
Chicago: Again, I’ve only ever seen the movie, but I love it. Great songs, sexy as hell (especially Catherine Zeta-Jones), and a great, cynical plot that is more relevant today than ever, in our culture obsessed with sleazy celebrities. I also love the fact that there is no love story to bog things down, and that the lawyer is a completely amoral, ruthless, brilliant scumbag. (Some of my law school classmates who saw the movie were truly, seriously offended.)
Wicked: Great story (I love humanizing “villains”), and Idina Menzel and Kristen Chenoweth are wonderful on the original cast recording. We saw a live touring version that was staged brilliantly. I just wish the songs were a little better.
The Lion King: Worth it for the costumes and sets alone, but a bit cheesy and too long otherwise. Julie Taymor is a mad genius, though. I really hope the rumors are true and her next stage project is a Spider-Man musical with Bono and The Edge writing all the songs.
Shows I don’t like:
Grease. We’ve seen and heard it all before, and it wasn’t that great to begin with.
Cats. Love the animals, hate the show.
Hair. I don’t care for hippies or hippie culture, and this show makes me tune out in a big way.
The Fantasticks. Seriously, fuck that show. It is my parents’ favorite musical, and they played the original cast recording (a record album, of course) constantly when I was a kid. Parts of it really freaked me out, and it’s just dated and boring and cringeworthy with all the rape references.
The Producers. I was a huge fan of the original Mel Brooks movie with Zero Mostel and Gene Wilder, but the musical was just too long, too sappy, and all the new material didn’t quite work for me. Again, I’ve only seen the movie, but wouldn’t spring for going to the show because I disliked it so much.
Least (since that was asked first):
HAIR - I can only assume that everyone was high. EVERYONE. The writers, performers, crew, and most especially the audience. It’s the only explanation I can come up with. A whole lot of drugs. Just hate it. (I feel the same way about TOMMY, but not as much).
Carousel, the story and characters are just so awful that the songs that are good can’t redeem it as a whole. (A second in this category - Kiss Me Kate)
Sunday in the Park with George just doesn’t work.
Selected Favorites:
I guess I’m among the few here who loves “Sweeney Todd” and walked out singing “It’s priest, have a little priest” and cracking up.
The Music Man
Guys & Dolls
Ragtime
I can’t speak for the audience, but from what I’ve heard about the writers, performers, and crew, yeah- pretty much. Two of the writers (Rado and Ragni) were the stars and Rado (the one who’s still alive) is still an old pothead.
I’ll just list my ***least ***favorite, since the other list would be way too long.
First of all, anything by Andrew Lloyd Weber. One good (or even great) song in each show, and the rest is repetitious, repetitious, repetitious, repetitious, repetitious, repetitious, etc. Especially Sunset Boulevard, which turned a great movie into trash.
(And I might add, here, anything that Patti Lupone is in.)
Rent. The characters are unlikeable, not even pitiable. The music is unmusical. Give me Puccini any day.
A lot of the George & Ira Gershwin shows: Of course we all love the great songs . . . but aside from them, the rest of the shows were filled with meaningless tunes and embarrassingly godawful lyrics.
And I don’t know what it is, but I find myself getting annoyed by a lot of Kander & Ebb music.
Basically, if a show is pre-mid-'60s, chances are I love it. Post-mid-'60s, chances are, I dislike it.
I’m a huge Sondheim fan, but the one show I can’t listen to is “Anyone Can Whistle.” It has one great song (“Everybody Says Don’t”); the rest is just irritating.
And those of you who can’t remember any songs from Sweeney Todd: how about “A Little Priest” and “Not While I’m Around.”
Thanks, now I have my earworms for the day. 
Manoel Felciano’s “Not While I’m Around” in the 2005 Revival was one of the sweetest, most heartbreaking performances I have ever witnessed.
never mind
Favorite: Little Shop of Horrors. If I ever got a chance to be in it (not that I deserve one), I’d love to be one of the girl singers. I guess I could accept the part of Audrey too. (Not Audrey Two). 
Least favorite: Cats. Why couldn’t y’all have told me it sucked? All I ever heard was how great it was.
…or “Johanna.” Or, actually, “The Ballad of Sweeney Todd,” which is awesome. Or “Kiss Me.” Or… you know, I find the whole score memorable, so this could take a while
OK, here are mine:
FAVORITES
Sweeney Todd: He had me at “there’s a hole in the world like a great black pit / and it’s filled with people who are filled with shit”
Ragtime: I want to stage that opening number so badly.
Parade: Best. Show. Ever. Kind of depressing, but the musical achievement is pretty extraordinary.
The Last Five Years: “Everyone tells you that the minute you get married / Every other woman in the world suddenly finds you attractive / Well, that’s not true. / It only affects the kind of woman you always wanted to sleep with / But they wouldn’t give you the time of day before / And now they’re banging down your door / And falling to their knees / At least that’s what it feels like / Because you / Can / Not / Touch / Them.”
For the Glory: This is the Frank Wildhorn musical about the Civil War (it used to be called “The Civil War”). A lot of Wildhorn’s stuff is pretty bad, but this is not.
Little Shop of Horrors: One of the best experiences of my theatre life was doing the puppeteer work for a production of this. So much fun.
Aspects of Love: For those who don’t know the trashy wonder that is this show, Bricker’s synopsis is incomplete: it’s actually about a young man who falls in love with a young woman who falls in love with young man’s much older uncle, who is himself involved in a relationship with a bisexual Italian sculptor, and the young woman and the uncle get married and have a daughter, but right around the time the daughter would have been conceived the young woman ALSO has a tryst with the young man, so when the young man comes back into everyone’s lives and falls in love with the uncle and the young woman’s 15-year-old daughter, he is macking on his first cousin or, very possibly, his own daughter. And then the Italian sculptress gets it on with both the young man AND the young woman, and the young woman ALSO gets a boy toy, and the uncle up and dies in the midde of all this.
I could go on in this vein, but the OP asked for shows I don’t like, so:
Oklahoma: The only musical in the world that makes me wish for death. The writing, the plotting, the absolutely ridiculous characterizations - all set to the soundtrack of my eternal damnation.
Passion: At some point, Stephen Sondheim must have decided he wanted to play a prank, and write one very long, very boring song, call it a play, and see if his rabid fans would pretend to love it. Insanely awful.
Hair: Inexplicable.
Wicked: I hated this show. I hated all the characters, except Kristin Chenoweth and that was only because she’s kind of hot. I wanted a brick to fall on Fiyero’s head. I wanted Idina Menzel to stop twitching and screaming and act. And then I left, and discovered that everyone on Earth apparently loves this show. So what do I know?
The Light in the Piazza: Really? Really?
Thoroughly Modern Millie: Stupid. Just stupid.
Legally Blonde: The Musical: I actually spent $240 of my own money on tickets to this for me and my wife. I left two hours older and considerably stupider, and with a compulsion to yell at people.
Ok
I can’t stand The Music Man. I actually like most of the show but I HATE BABERSHOP QUARTET. HATE HATE HATE.
I never even saw Ragtime because I can’t listen to more than 30 seconds of Ragtime music without my head starting to explode. Also Ragtime is always a complete earworm for me.
When I finally saw Les Miz, I was completly underwhelmed. The cast looked like they were sick of the show. So while I like most of the music, I don’t like the show.
Shows I love.
42nd Street Gosh Golly it’s a fun show.
Kiss Me Kate
Man of La Mancha
and probably the best show I have ever seen on Broadway is
The Lion King
The costumes, the staging, the (added) music, it is really just spectacular.
I’m not ignoring them. They are dull and uninspired, and really badly cliched. (“Soliloquy” is the most cliched and tuneless song ever written for Broadway). The score is nowhere nearly up to the level of Oklahoma, The King and I (overall, their best), South Pacific, The Sound of Music, (which I’m not a fan of otherwise) and even Flower Drum Song (quite underrated).
<kills self>
I think we may not be agreeing on what precisely is meant by the word “score.”
Least Favorite:
Favorites
Oklahoma and Cats. Especially Cats. Seriously. I can’t even imagine what Eliot would think if he could come back from the dead and see that atrocity. Wasn’t a huge fan of Phantom of the Opera either.
There was also this GODAWFUL musical version of The Tempest that I saw in Korea. When the local newspaper called it “a work that will appeal to everyone, Shakespeare fans or no” I was furious. I mean, they dressed Ferdinand in shiny white pants, like he was entering a figure-skating competition or something. Ugh.
My favorites would be
Hedwig and the Angry Inch
Children of Eden
Chicago
The Sound of Music
My Fair Lady
The Last Empress
In defense of Frank Wildhorn…
The show did not open with Hasselhoff in the role. The role was done on Broadway first by Robert Cuccioli, who was amazing. The versatility of his transformations between J and H was out of this world. In contrast, Hasselhoff seemed to feel that fliiping his head from one side to the other and getting a different color key and fill light scheme on him constituted “a new character.”
Other greats in the original Broadway production were George Merritt (of “Big River” fame, who signed my Playbook!) and Christiane Noll, who was also an amazing Ellen in Miss Saigon.
So while it’s not perhaps an “all time” great, I think it deserves a bit more praise.
I would so love to see a dinner theatre production of Sweeney Todd. What a mesmerizing show.
Like others from the Midwest, most of my exposure has been via cast recordings, touring productions or community theater. There was a time when I was without a TV for several months and dove into Broadway musicals HARD. One of these days I will see those shows that kept me entertained.
Least favorites:
I saw a production of Beauty and the Beast here that was PAINFUL. The songs are meh (except for Be Our Guest).
Gypsy - while the story is compelling, having Rose be the focus just really makes it an ode to stage mothering which, on principle, I abhor.
*Grease * - really, really. No plot. Uninspired pastiches of 50’s tunes.
Others: Carousel, Wicked
Faves:
Jekyll and Hyde: saw it in Chicago on the way to Broadway and was STUNNED. If you’re on the fence, grab the 1994 concept cast recording.
*Assassins * - both incarnations.
A Chorus Line - mostly because that is the first professional theater production I remember seeing.
Into The Woods
Passion - I haven’t seen it but the music pulls me and breaks my heart.
Sweeney Todd - I have been atwitter for months waiting for the movie to open.
Others: Le Miz, Oklahoma, South Pacific, The Music Man
The WHA HUH?
Eating Raoul: The Musical - there is nothing wrong with Adrian Zmed singing about Hot Monkey Love.
The menu just writes itself!