Gee, our old LaSalle ran great!
For me, right now, it’s a tie between “Just Communication” and “Rhythm Emotion” from Gundam Wing.
For Western series, it’s the DS9 theme.
Oh, and I have to mention “Zankoku na tenshi no teze” (Cruel angel’s thesis) from Neon Genesis Evangelion; the theme from the British Queer as Folk; and Michael Nyman’s sublime “The heart seeks pleasure first” from The Piano.
Jack Spade’s theme music from I’m Gonna Git You Sucka.
Duel of the Fates was amazing. Sadly the movie was so terrible that I only learned to appreciate the song a few months ago while plays Obi-Wan on Xbox.
Also a few that have already been mentioned: Jurrasic Park, Sanford and Son, Mission Impossible, Star Wars.
While not officially the theme music, the ‘dueling banjo’s’ from Deliverance always struck me as strangely appropriate to the tone of the movie.
And I just like banjo’s anyway!
D’oh, I didn’t even think of anime themes until matt mentioned Eva. The themes to Trigun and Cowboy Bebop are awesome.
Quoth RaCha’ar:
Did your percussionists do the raptor call thing, too? There’s some way to run your fingers over the surface of a timp that sounds exactly right.
And on Star Wars, the obvious “Imperial March”, main theme, and “Battle of the Fates” have already been mentioned, but my favorite is the Old Republic Theme (when Leia is handing out the medals). It really captures the nostalgia.
Does it show that I’m a fan of Williams?
I adored Braveheart’s theme song… and the love soneg " For the Love of a Princess"… they also used that in Castaway. - anyone else notice?
I’ll agree with many of the above and add Perry Mason
Fever, from Dr. Mario
I probably haven’t played Dr. Mario in over a decade but I still hum Chill outloud almost daily.
How could I have forgotten Twin Peaks?
The best theme song ever from a movie, and my friends and I have actually had this conversation (yes, we’re lame) is the Storm Troopers theme (is it called The Imperial March?) from Star Wars, and I’m not even a big Star Wars fan. You hear that music and you know something’s a-coming for you, and it’s gonna kick your ass hardcore.
Also, that sad, sad violin music at the end of Schindler’s List is beautiful, and makes me cry like a baby every time.
Besides what’s been said, how about the theme from Back to the Future?
Ennio Morricone’s:
‘The Good, The Bad and The Ugly’ theme.
Also by Morricone ‘On Earth As It Is In Heaven’ and * ‘Gabriel’s Oboe’*
both from ‘The Mission’
Also - no one has mentioned Carl Orff’s ‘O Fortuna’ from ‘Carmina Burana’ used in a million movies, commercials and computer games.
I think I’ve said this before, but: the theme from the late fifties TV show “M Squad” done by Count Basie (and riffed on for the “Police Squad” parodies).
Best theme ever, no contest: Space 1999. It has everything!
Also notable are both themes to Lost in Space.
And I want to third the votes for Cowboy Bebop. That entire score is beyond brilliant. Yoko Kanno is my hero. My favorite track is “American Money,” the theme from the bounty hunter show.
Ooops… I put my movie picks in this other thread. Let’s just say I’m sort of a film score buff. Waiting anxiously for my order of Horner’s score for Iris as I type. 
Since people here are mentioning TV and computer game themes, though, I’ll throw a couple of those in.
For television, I always loved the theme used in the orignal Star Trek episode, “The Doomsday Machine.” As far as I’m concerned, John Williams ripped the idea for Jaws right out of that theme.
DUH-nuh, DUH-nuh, DUH-nuh… shiver Relentless. Very evocative.
And for computer games, the music for the PC game “Homeworld” was a stunning piece of work. It incorporated a choral version of Samuel Barber’s “Adagio for Strings” (also heard in Platoon), an original song by Yes written for the game, and some amazing battle music and ambient space themes. Great stuff. The “Game of the Year” edition comes with a soundtrack CD, check it out!