Gonna show my age here, but I have to go with Monty Python as a step above anyone else. And on a somewhat(ok, much) lower level I really loved Benny Hill- just looking at his face made me laugh, though I couldn't understand about 25% of what he said- especially in the songs. And others have mentioned Carol Burnett- pretty good if you could stand to watch the whole show, though Harvey Korman was amazing. And I have to mention Tracy Ullman too, she's a tremendous talent, though I don't know if she fits in this category.
Can't really comment on most of the others- don't really watch tv anymore, mostly because I'm too cheap to pay for it, and if I really want something I'll have one of my sons record it for me. Matter of fact, one of my sons did give me a dvd of Kids in the Hall which I've never watched, guess I'll have to check it out.
And as for the question of why, it doesn't matter why or how much you explain it, you either like it or you don't.