John Oliver is pretty much the undisputed king when it comes to "quality," right? I mean he's the guy you read stuff about how he's enlightening the masses and all that jazz. He deserves a lot of credit for coming up with the concept that he did. It lends itself to strong shows and he and his staff make the most of it. Doesn't hurt that he and his writers consistently do produce material that actually is also really funny.
Sam Bee seems like the basic cable version wannabe John Oliver. I respect her and what she's doing, but I don't like watching her.
Sometimes I can hardly believe Jon Sterwart stumped for Noah to get the gig when he left. Noah's charming as hell, very charismatic, but he's not a good comedian (IMO), which translates to a really bad Daily Show. Larry Wilmore would have been so much better, such a raw deal the way that whole thing turned out.
Conan O'Brien reminds me of Letterman in that he can make any interview entertaining. Really witty, really smart. I think he's the most naturally funny of all of them, and that comes across on his show.