Pick your top 3. These were the 10 best that came to mind (in no order), I'm sure I'm leaving something good out. I'm leaving SNL out because it's been on so long it would need a thread of its own with the various casts. Also, no Monty Python or Chappelle, because only an idiot wouldn't vote for them.
i've never heard of 90% of these shows. also where shapelle show? edit: just read the op. i is eediot.
Mad TV at the top for me, close second In Living Color. Both very quotable. UCB is a great dark horse. Honorable mention: Benny Hill
whose line is it anyway has to be too 3 tbh. it's improv, but close enough, since most of their games are sort of pre-scripted to fit the breh's strengths
Mad TV, In Living Color and Mr Show from the list. Missed Key & Peele but I've seen some of the sketches here and there. On the British side, I'd throw in That Mitchell and Webb Look for fans of Peep Show....