Um... that's kinda hard to say... as, uh... They both have their "moments"... but I was never one for "silly"...
But I'd have to go with the new Mikey. Even though he sometimes gets to be a little much, he's alot funnier. Especially since he uses alot of pop culture references...
EDIT - Sometimes I think that the New Toon Raph is just as funny (if not funnier) as Mikey, with some of the comments he makes. And Leo and Don both have their funny moments, too.
Then again, it all depends on your definition of "funny", and your own sense of humor.