i think every one except Leno is utterly watchable. I never though Leno was particularly funny but he was a reasonably safe, affable host for the Tonight Show. Since the Conan debacle, i find him neither affable nor watchable to be honest. and the thing is- like heavy hands said- i'm sure in certain respects he's a nice guy. But i just can't put the Late Shift stuff and the Conan/NBC fallout out of my mind when thinking of him.
as for the others
Fallon- he started out SO uncomfortable, nervous, and awkward that it made me root for him to get better and I think he has. He's still the weakest of the non-Leno names here but he's developed much more of a groove- his monologues are no longer unbearably forced but actually natural and incisive and he interacts well with guests. He also has an epic house band in the Roots.
Kimmel- Kimmel and Ferguson are kind of the dark horses in this race. Jimmy is just a naturally funny guy and he has great delivery, timing, and also comes across as an everyman, likable character. I think he's just overall fun to watch and while I think some of his segments are misses (like any late night show) the vast majority of his stuff works very well.
Letterman- a personal favorite of mine throughout the late 90s early 2000s and then again when i was in grad school in the years before the second decade of the 2000s, I just find Dave to be a funny guy. He's not the same level of funny that he was on NBC like thirty years ago but he's now curmudgeonly funny which I think has its own value. The top 10 lists can still be great to watch, his interaction with Shaffer is still awesome, and i love some of his segments- stupid pet tricks, stupid human tricks, fun facts, etc. His delivery during fun facts and some of the comments he makes after reading the cards are awesome.
There was also the hysterical Great Moments in Presidential Speeches that of course ended with Dubya's administration coming to a close. He can come across as a jerk and a bully sometimes (his interviews with Bill O'Reilly from a few years back, for instance- I know a lot of people feel O'Reilly is an ass so he got his comeuppance but I think Bill came off well, trying to make things light and humorous but Dave was just incessantly in attack mode. you're not a political commentary show, Dave. if O'Reilly is going to be funny, go with the funny).
Still I love Letterman.
Ferguson- underrated as fuck in my view. Here's another guy who just naturally projects a funny vibe. I love the way he tells anecdotes and that he doesn't necessarily follow the trend of quick joke, quick quip, quick joke. At times he'll take his time working his way to a punch line. I haven't watched his show in a long time now but it's not for lack of wanting to. He's definitely one of the best at this point.
Conan- His humor isn't for everyone, I guess, which is why NBC was oh so quick to can him but for those who are devoted Conan fans, he's always been stellar. In my view, a guy like Leno just doesn't have as much off-the-cuff comic ability as Conan. And while Conan was a writer by trade and Leno was a stand up comic, I think that Conan just projects the more funny, spontaneous ability. His responses to guests are awesome. His self-deprecating humor is fantastic. His string dance is epic. Some of his past running gags were amazing- sexy manatee website being one of the best of all time in my view. I havent seen much of the TBS show sadly but when i check out clips on youtube, i can see he's still definitely on his game.