Really tough call. As funny as they both were they're quite different. I think Groucho wins by a hair by virtue of his quick wit and sharp delivery but Curly's slapstick antics keep him in the game.

The Marx Bros. screamed ANARCHY and they made you feel like you were run over by a steamroller (if you were one of their "victims"). The 3 Stooges were sillier but they made too many lame movies (mostly in the '40's and '50's). Their early Columbia shorts (made in '34-'36) were hilarious and of note was a 1940 short in which they did a takeoff of Adolf Hitler but they faded into moronic, boring movies after that (especially after Curly had his stroke). The Marx Bros., on the other hand, made some truly wonderful flicks in the early-mid '30's but also faded somewhat after that.

I'll give my vote to Groucho and brothers. (Their stateroom scene in "A Night at the Opera" is probably the funniest single scene I've ever seen in a movie).