My 2 cents:
Leno - not funny, annoying. Jay Walking making fun of dumb people, not a nice thing for a guy who is supposed to have a nice guy image.
Letterman - sometimes funny but lately seems like he's phoning it in, not really into it. When he's into it, he's great. Not really good at interviews.
Conan - can't wait to see what he does next. Thought he was good on Tonight don't care for the crude stuff like the dog or the bear. He does a good job of interviewing. Check out his interview with Shatner and Hayden Panitier (sp?). He and Andy worked well together and I enjoyed "Twitter Tracker".