I've always liked Fallon, but his talents are much more on display when he's doing singing impressions of Jim Morrison or Neil Young. Granted, that's a tough thing to build a whole show around.
Leno is just a stiff. I have never found him funny. I am completely at a loss as to why he appeals to people. For my money, Craig Ferguson is the one currently hitting the late-night talk show ball out of the park.