I can spend hours at the TV Tunes web site.

I think most of the best & most memorable themes were instrumentals:

Hill Street Blues
St. Elsewhere
The Odd Couple
L.A. Law
Buffy the Vampire Slayer
Hawaii Five-O
Magnum PI
The Rockford Files
Taxi
MacGyver
The A-Team
Simon and Simon
M*A*S*H
The Avengers
Barney Miller
Dallas

Some of my favorites with lyrics:

Cheers
Ally McBeal
China Beach
Tour of Duty
The Mary Tyler Moore Show
Spiderman (1967)
Married With Children