I’ll toss a vote in for The Blues Brothers. Aside from being a comedy, it’s got a fantastic collection of music, and really captures the essense of a certain kind of musical culture.
Also, Singing in the Rain, or most of the musicals on this list.
Others may disagree with this, but…
It wouldn’t have occurred to me if I hadn’t seen the company’s name on another thread title, but the Disney movies probably deserve a place of honor on this list. Taken as a whole, they probably constitute the best known group of musicals in American history. Most people who would never dream of seeing a musical could probably still hum along with songs from everything from “Snow White” to “The Lion King.” Not sure exactly which specific soundtracks are the best, but “The Jungle Book” is a personal favorite.
Well, I don’t know about ‘as a whole’… those movies have composers too. 
My favorite is probably Alan Menken (who’s work has already been mentioned here in the form of Little Shop of Horrors).
He did Aladdin, which is another great soundtrack/score (as well as many other Disney flicks; Beauty and the Beast, The Little Mermaid, etc etc).
The Lion King also has great music, with the songs by Elton John and Hans Zimmer doing the orchestral score (IIRC).
Braving the onslaught and offering forth that everyone is entitled to their own opinion, I will uncategorically state that no greater schlock has ever been thrust upon humankind than the crap spewed forth by Andrew Lloyd Weber.
End of venting
Now to get on to the matter of other noteworthy, but I believe previously unmentioned, nominations for a host of reasons:
[ul]
[li]The Harder They Come[/li][li]Threepenny Opera[/li][li]*Hard Days Night * & Help[/li][li]*Eddie and the Cruisers * (John cafferty & Beaver Brown) [/li][li]*Ascenseur Pour L’Echafaud * (Miles Davis)[/li][li]*Anatomy of Murder * (Duke Ellington who also appeared as "Pie-Eye)[/li][li]Seven with selections from Thelonious Monk, Billie Holiday and Charlie Parker[/li][li]*Les Liaisons Dangereuses * (Roger Vadim movie with music by Monk and Blakey)[/li][li]Round About Midnight[/li][/ul]
The latter five are a nod toward the contributions made by jazz musicians of which there are others that I cannot think of at this moment.
OBC of Ragtime- it’s a musical for people who don’t like musicals.
Broadway Revival Cast Recording of Cabaret (much better than the original [which starred Bert Convy, for Og’s sake], though if money’s not an option I’d get the movie soundtrack just for Joel Grey)
Nine- mostly forgettable, but “In a Very Unusual Way” and “Bells of St. Sebastian” are hauntingly beautiful- I like the Antonio Banderas version better than the Raul Julia.
My next theater soundtracks will be Dirty Rotten Scoundrels and the revival cast of Flower Drum Song.
Personally I like Sunset Blvd., but not everybody does. There are several versions, but my favorites are Patti Lupone and Glenn Close (and favorite song is As if We Never Said Goodbye- it’s actually disappointing watching the same scene in the movie after hearing this song, in which Norma almost has an orgasm in song when the spotlight hits her for the first time in 20 years- in the movie the scene lasts 5 seconds.)
And I can’t believe that Annie Xmas, of all people, didn’t recommend ALW’s Whistle Down the Wind. 
I know it’s too late, but I’d strongly encourage you to split “Film Music” from “Film Musicals/Stage Musicals”. The two genres (IMO) have nothing in common. One provides backround music and (often essential atmosphere) to films, the other tells a story with song.
That said, here’s my (too long) list of musicals. I’m ignoring operettas and limiting myself to no more than 3 musicals per team just for the helluvit. [ul]
[li]Irving Berlin-Annie Get Your Gun (Revival, Kim Crisswell), Call Me Madam (Original Soundtrack w/ Merman)[/li][li]Jule Styne/Stephen Sondheim-Gypsy (Original Broadway Cast–no one will EVER do Rose better than Merman)[/li][li]Stephen Sondhem: A Funny Thing Happened On The Way To The Forum (Original Broadway Cast), Sweeney Todd (Original Broadway), Saturday Night (Original Broadway Cast–not really essential, but a lot of fun)[/li][li]Bernstein: Candide (Recent revival with Kristen Chenoweth), West Side Story (either Original Broadway Cast or Original Soundtrack–the singing’s better in the original, the orchestration is much better in the Soundtrack)[/li][li]Comden and Green: Bells Are Ringing-Original Broadway Cast, even though Dean Martin has fun in the OST.[/li][li]Loesser: Guys And Dolls (Original Broadway Cast), How To Succeed (Original Broadway Cast), Greenwillow-Not essential, but fascinating (Original Broadway Cast-with Anthony Perkins!!!)[/li][li]Lerner and Lowe; My Fair Lady (Original Broadway Cast-Mono Version: the Stereo version was recorded a year later and the performances aren’t as fresh), Camelot (Original Broadway Cast)[/li][li]Webber: Evita (Original Concept Album w/ Julie Covington), Jesus Christ Superstar (Original Concept Album w/ Murray Head)[/li][li]Stephen Schwartz-Baker’s Wife (Original Broadway Cast), Pippin (only the one cast exists) and Wicked (Original Broadway Cast)[/li][li]Kander and Ebb: Cabaret (Original Broadway Cast) or (Original London Cast-with Judi Dench–ha. IF you can find it) and Soundtrack. They’re really two different musicals that share some songs in common.[/li][li]Schmidt & Jones: The Fantasticks (Original Broadway Cast)[/li][li]Bock & Harnick: Fiorello! (Original Broadway Cast), Fiddler On The Roof (Original Broadway Cast)[/li][li]Hamlisch & Kleban: A Chorus Line[/li][li]George Gershwin: Of Thee I Sing/Let 'em Eat Cake (1987 Studio Cast), Porgy And Bess…um…pick your favorite. There’s too damned many good ones.[/li][li]Kern & Hammerstein: Show Boat (1987 Studio Cast on AEI Label–they did their best to recreate the show as it originally was performed–and have wonderful performances.)[/li][li]Adler & Ross: The Pajama Game (studio cast by Jay Records), Damn Yankees (Original Broadway Cast)[/li][li]Merideth Wilson: Music Man (either Original Broadway Cast or OST–both have things to recommend them. I prefer Original Broadway Cast 'cause I really prefer Barbara Cook to Shirley Jones and Preston’s performance is marginally better in the Original Broadway Cast)[/li][li]Rodgers & Hammerstein: Oklahoma! (1979-ish revival Lawrence Guittard(sp?) is a Curley that doesn’t sound like he’s 40), King and I (1995-ish Studio Cast with Ben Kingsly and Julie Andrews), Carousel-I’m not sure which cast. Possibly the OST.[/li][li]Strouse & Adams: Bye Bye Birdie (TV Soundtrack with Vanessa Williams), Annie (Original Broadway Cast)[/li][li]Mitch Leigh: Man of La Mancha (Original LONDON Cast)[/li][li]Sherman Edwards: 1776 (Original Broadway Cast)[/li][li]Menkin & Ashman: Little Shop Of Horrors (Original Broadway Cast, or French Cast if you can get it)[/li][li]Lionel Bart: Oliver! (Original London Cast)[/li][li]Jerry Herman: Hello, Dolly! (Original Broadway Cast)[/li][li]Schonberg: Les Miserables (Original London Cast)[/li][li]William Finn-A New Brain (OBC)[/li][li]Vincent Youmans: No No Nanette (1971 Broadway Cast)[/li][li]Sandy Wilson: The Boyfriend (1954 Broadway Cast w/ Julie Andrews)[/li][li]Lane & Harburg: Finnian’s Rainbow (Either 1960 Studio Cast w/ Jeannie Carson OR Film Soundtrack with Petula Clark)[/li][li]The ABBA Guys and Tim Rice: Chess (Original Concept Album-With Murray Head and Elaine Paige)[/li][li]Stephen Flaherty & Lynn Ahrens-Once On This Island (Original Broadway Cast), Lucky Stiff (not essential, but very, VERY funny) (Studio Cast on JAY label)[/li][li]Cy Coleman: Sweet Charity (OBC), On The Twentieth Century (OBC)–not essential, but great[/li][li]Rodgers & Hart: Babes In Arms (1998-ish Broadway Revival Cast), Pal Joey (1952 Revival on Angel Label or 1995 Concert Cast)[/li][li]Kurt Weill: Lady In The Dark (Rise Stevens cast–which includes bonus Danny Kaye tracks!), Threepenny Opera (I like the Raul Julia cast from the mid '80s, but it’s not a traditional rendition)[/li]Wright & Forrest: Kismet (1965 Lincoln Center cast)
[/ul]
<nitpick>
The Fantasticks was never on Broadway. It opened and closed Off-Broadway.
</nitpick>
Don’t – you will be sadly disappointed. The changes are monumental from orchestration to song inclusion to settings – I had both on tape, when I bought my first CD player, the original concept was the only one I replaced.
Unless you’re a completist, I wouldn’t bother.
You’re right. That’s true of a few other musicals I listed as well. I’ve got a bad habit of using the phrase “Original Broadway Cast” for “Original Off-Broadway Cast” or “Original American Cast”. IIRC, The Baker’s Wife is also an Off-Broadway Cast.
Also, I agree with everyone else who’s commented: the OBC of Chess is pretty feeble (although the female lead has a pretty voice). Get the Original Concept Album with Paige and Head for best listening pleasure (IMO, clearly the best cast) or if you prefer completeness of score over vocal/acting ablitiy, the rare version of the Danish Cast with the four(?) songs not included in any other version (Two of 'em are (from memory, so the titles may be off): “Commie Newspapers”, “The Soviet Machine”).
I don’t know if A Charlie Brown Christmas counts as a movie, but the soundtrack to that by Vince Guaraldi is fantastic. And if the Looney Tunes shorts count as movies, then any Carl Stalling recording deserves a nod.
In case someone doesn’t think ALW is a (rich, successful) hack, here’s my recommendations for each of his shows:
The Likes of Us: ALW and Tim Rice’s first show, recently released on CD. Good for historical value.
Joseph and the Amazing Technicolor Dreamcoat–Well, the original version was on CD for about 12 seconds, but it’s very expensive now. The OCB with the late Laurie Beecham is my favorite. For foreign language versions, the German cast is spectacular, and the Italian very, very good.
Jesus Christ Superstar–Original Concept CD with Murray Head as Judas. It has never been equaled. For a foreign laguage version, the Czech cast (either 1CD highlights or 2CD live) has the amazing Dan Barta as Judas, who won all sorts of awards.
Jeeves/By Jeeves–I love this recording, but it’s very British humour, and not to everyone’s taste. A few English recordings float around, with the video cast being the easiest to find.
Evita–Original Concept CD with Julie Covington and Colm Wilkerson. My favorite recording of any show. Foreign language, again the Czech CD with the amazing Dan Barta. There’s also an 11 song Tokoyo CD that is astonding, if short. And the movie soundtrack is pretty good.
** Song & Dance**–The 2CD Marti Webb version is now available, and the OBC with Bernadette Peters is pretty good. The German version is rare, but worth finding.
CATS–Get the OLC with Elaine Paige, which includes The Battle of Billy McCraw and the original Mungojerrie and Rumpletezer. Foreign language, go with the 1CD Austrian version for Michael Howe’s Rum Tug Tugger.
Starlight Express–The 2CD OLC is a good recording of the show, and the only complete English one. The 2CD live Bochum is a better one, even in German.
Phantom of the Opera–The OLC. Nothing better. Foreign language, the Swedish 2CD and the Korean 1CD (the Korean Evita is one of the worst things ever put on a CD).
Aspects of Love–The 2CD London cast is one of ALW’s finest. There has yet to be a good foreign language recording.
Sunset Boulevard–The Patti (Lupone) vs. Glenn (Close) debate rages on. I prefer the OBC with Glenn. Foreign language, the German cast with Helen Schneider.
Whistle Down the Wind–2CD OLC. This show sunk like a stone, and is one of the few I don’t have a foreign language recording of.
The Beautiful Game–Get both the OLC and the German cast recordings. One of ALW’s less known, but good shows.
The Woman in White–ALW’s latest, and I love this show. The OLC recording is good. I hope the Broadway cast gets a complete CD.