I was seriously trying to think of a legitimate answer, but I can't think of one female on television who makes me laugh.
Sexist, and sad. But other guys in this thread have said the same thing. That is kind of amazing given the funny women listed.
That I absolutely can not stand her, appearances on Curb notwithstanding. This would be my first pick. I haven't watched her show in a while, but her stand up is great. Tina Fey would be my second choice. I think if you were to open it up to funny female characters you would get more responses. The question as I read it is asking for the funniest non-character. It's actually true for men as well, I'm trying to think of inherently funny men and come up with fewer than I thought I would.
Mila Kunis. I love her. Have you seen Saving Sarah Marshal? Yeah. She's ridiculously hot in that movie.
I agree with that, actually. There aren't very many guys who are funny without playing a scripted character. Dave Chappelle is one of the few I can think of off-hand. Jon Stewart often makes the list, but less so these days as his efforts are becoming less cutting satire and more Vaudeville.