Can't just list 5, so here are my top 7 comedy shows:
Frasier - the best sitcom of all time, hands down. I'm currently watching this for the first time (on the DVD sets) and am on the 5th season.
Cheers - hilarious & iconic
Taxi
Night Court
Seinfeld
Curb Your Enthusiasm
It's always Sunny in Philadelphia - sick, twisted, and awkward - but also extremely funny.