I'm voting for Seinfeld, for some reason cruelly ignored by British TV, but without a doubt the best comedy show ever.
Plus they managed to get an episode solely about masturbation on prime time US tv, which scores bonus points. Curb Your Enthusiasm is splendidly nihilistic.
From the Brit side, Brass Eye was better than snorting uncut cake (though Jam was, erm, Weird). I never got into League of Gentlemen, it always seemed to try too hard to be weird. Spaced was very good.
I just don't get sketch shows where they just repeat the same sketches each week, with a minutely tweaked scenario.