Ferguson was very good and underrated, but a bit much if you were watching nightly. But I like him a lot, very quick and very engaging.
Leno was whatever, pure vanilla.
Carson was great, especially for his time. A sort of classy silly, which no one has really done since then. Classic for sure.
Conan was the best all around though. An obviously nice guy (I can attest in real life too, since I once met him and chatted for 5 minutes), also super self-deprecating, which is funny itself, but also just good at everything - allowing the guest to speak, perfect reactions, funny interruptions, very intelligent, extremely silly, and would basically be one with the audience (just look at when he starts laughing for real at guys like Norm, Martin Short, Sandler, and others - he's laughing just as hard or harder than us and it's genuine).
Dave was smart and entertaining or amusing, I'd say. But he was weird and offputting, a very awkward guy pretending to be normal, and wasn't really "funny" per se. And the non-interview stuff was just awful. I never once laughed at anything on those idiotic top 10 lists. He also isn't all that great a guy, and that comes through too.