Jay Leno is not funny. He must have had a hard go of it as a stand-up comedian because he is not funny at all.
Neither is David Letterman or Craig ferguson. Ferguson may very well be the single most annoying human alive today. Letterman's monologue and top ten lists are pathetic.
Jimmy Kimmel can generally make me laugh two or three times in a week's worth of monlogues. Jimmy Fallon will usually make me chuckle just about every time. I can't ever recall laughing at anything Conan has said on the occasions that I have heard him.