If we are talking themes written especially for the show, then probably
Peter Gunn,
West Wing,
The Virginian,
The Wild Wild West, and the original
Hawaii-Five-Oh. Oh, and
Sesame Street
Probably the most iconic and recognizable to the most people is the theme from
M*A*S*H*, but that wasn't written for the TV show; it was written for the movie. (If you have never seen the original movie, the same score was used, but it had lyrics. The title of the song is "Suicide is Painless" -- which makes this song's popularity at weddings rather ironic.)
Now then, if we talk about best title sequence, that's another story. My rock-solid vote for that one is
China Beach (the Supremes'
Reflections), followed by the original
Hawaii Five-Oh, and then another Vietnam drama,
Tour of Duty (the Rolling Stones'
Paint it Black). The Wild Wild West also had a good title sequence, it was animated. Oh, also honorable mention to
Get Smart, because it's funny.
China Beach title sequence:
http://www.youtube.com/watch?v=7WcatSNjvVM
Original Hawaii Five-O title sequence:
http://www.youtube.com/watch?v=ht3Al9H3iUM
Tour of Duty title sequence:
http://www.youtube.com/watch?v=0z6fcd--354
Wild Wild West title sequence:
http://www.youtube.com/watch?v=8qbim6I51G4
Get Smart:
http://www.youtube.com/watch?v=ElqZms_SUjg