I love the theme music from “Dr Who.” It evolved over the years, and was always compelling.
My favorite instrumental is the theme from the original Battlestar Galactica. Very moving orchestral piece. My brother and I couldn’t help conducting during the opening to that show, back when it first aired.
I once saw David Duchovny on a talk show, and he let out a secret: The X-Files theme had lyrics, written by the composer, Mark Snow. It goes like this (you’ve got to hold the instrumental theme in your head as you read):
“The X-Files is a show…
with music by Mark Snow…”
Of recent shows, my favorite I think is Firefly.
Least favorite would have to be Joan of Arcadia. I didn’t like the song on the radio and I like it less on TV.
The Fall Guy.
Can’t think of a least favorite theme (except every show on the FoxBox–I’ve had the Sonic X theme stuck in my head for days and it’s driving me insane) but the rabid fangirl in me has to chime in with Inner Universe, the Ghost in the Shell series theme. The song is really awesome just to listen to. I also looked up a translation once, and it really fits the show (though who would know–most of it is in Russian). Add in the CGI opening and it’s the best opening title I’ve ever seen.
I second the Rockford Files for one of the best, and add one of my favourites : the Doctor Who theme. So bizarre, yet catchy.
The Greatest American Hero
The Beverly Hillbillies
And the absolute best theme song of all time…The Mysterious Cities of Gold. Bow before its glory.
The new reality show on A&E called Intervention.
Haven’t even seen it yet, but anything that uses BareNaked Ladies’ What a good boy wins! 
Wow, I never get to post to these things because someone always beats me to whatever choice i have.
Anyway: best hands down, instrumental or not, Barney Miller. I will not argue about this. Second is Good Times.
Worst: how 'bout Eight is Enough? If you can’t tell already, I stopped watching TV in the 70’s so I dont know about all those hideous-sounding shows like Saved by the Bell and Full House and Spunky Brewster and stuff. I bet those were pretty horrible, but could they be horribler than Eight is Enough?
It’s hard not to be happy when The Bob Newhart Show comes on.
The BBC series “McCallum” had a great theme song with (I think) Julie London singing “Cry Me A River”; very moody, over an overhead shot of London at night.
“Twin Peaks” and “The X-Files” are two of my favorites; they both captured the essence of their respective series. The same goes for “Angel.” And “Beverly Hills, 90210.”
As for theme songs with lyrics, like others I adore “The Greatest American Hero.”
Lots of good choices above, but the theme from The LIfe and Times of Grizzly Adams was great. I wish they would have recorded an extended commercial version of the song.
A musician friend with an expansive audio system once parsed the theme from “Mission Impossible” down to the bongo. It really is the heart of the theme song, not the claves that one usually recalls.
Johnny Quest’s music really deserved a better gig. During regular working hours, the Hanna Barberra musicans laid down the Deputy Dawg’s tracks. Later they’s send the piccolo guy home and, after a few blunts, recorded Johnny Quest’s.
First two I thought of were The Rockford Files and Twin Peaks.
Two more I haven’t seen: Simon & Simon and The Avengers.
Can’t think of any really bad ones.
Too many contenders for the best, but the theme from The Price is Right irritates the shit out of me - I’d rather have railroad spikes pounded into my ears than listen to it. Is the freakin’ show even on anymore?
Probably from how you spelled his name. 
ST:Enterprise is a yicky, yicky song.
Everwood’s theme fits the show.
I’m a sucker for cello so the themes from Angel and Forever Knight are among my faves. What is it with vampires and cellos?
Has anybody mentioned The Addams Family yet? Or headed over to www.retrocrush.com where they are running a Top 100 TV Theme Songs special?
I think the very first thread I started here on the Dope Board was a rant that the theme music for The X-Files was just a ripoff of the theme music to the 1960s Eve Arden/Kaye Ballard sitcom The Mothers -in-Law , only played a ittle slower and more spookily. Now I know who ripped it off.
I gotta admit, I like the instrumentals of the 1960s, too, esecially Hawaii Five o and Mission: Impossible and Mannix (Weird trivia – Lalo Schifrin, who wrote a lot of TV themes, including MI and Mannix, was a sociologist. My mom’s boss, a sociology professor, knew him.)
I like Alfred Hitchcock’s theme song, too. It was years until I learned that it’s a odd piece of classical music, not an original. It’s Gounod’s Funeral March for a Marionette.
Best without lyrics: Mission: Impossible. Honorable mention to *The Equalizer, Simon & Simon, Hill Street Blues, * and Lost in Space (2nd version)
Best with lyrics: Secret Agent Man. Honorable mention to The Jeffersons, The Fall Guy, Animaniacs and Spider-Man.
I would add Angel’s, too.
A lot of good ones already mentioned. Here’s a few more.
My Three Sons
Hill Street Blues
Highway Patrol
Hogan’s Heroes
Route 66
Law and Order