Since i'm in Germany i don't get US Late Night chats but then most of it is on Youtube and i like to watch them occasionally for fun.
Personal favorite is Craig Ferguson by a long shot.. that guy didn't give a rat's ass about political correctness or not roughing some feathers of A list celebrities (when he had some on the show). He was just there to have some fun and i think most guests understood that and were much more relaxed than in other talks, I also rarely got the impression that the chats were pre-scripted. Of course many celebrities were there to promote their current project but often enough it was a smaller segment of the entire chat and the rest was just going wherever it went which was hilarious most of the time. Ferguson's charm also took out the sting from his remarks and turned it into something funny and if he had a guest who could keep it up was brilliant.
Next in line would be Corden. He seems genuinely having fun and happy to have scored this job and he rarely holds back his glee. Carpool Karaoke is probably the best thing to ever come out of the show or late night chats in general and it's been so much fun to watch them. His other games are funny too, i just saw Spill your guts with Mila Kunis and Christina Applegate and it's hilarious. However he sometimes tries a little bit too hard and it shows when he overreacts and laughs his ass off a little bit too much.
Third would be Conan who can be very funny too when he's not checking off the scripted parts of the talk. Whenever he goes off the trail it's funny as hell.. especially his Office videos (i love Sona

) and his bits with Ice Cube and Kevin Hart are utterly fantastic and funny because there is no script and they just let loose. Bad thing is his utterly ridiculous fake laugh and you can always tell.. i wish he would improvise more but then again it doesn't seem to be his style and he seems to be content being a part of the big Hollywood promotion machine with the occasional burst of the real Conan. However there's one video with comedian Bill Burr where he rips into Oprah and Lance Armstrong and you can see Conan totally lose his shit.. one of the few instances where he is actually really laughing.
Bottom rung are Kimmel and Fallon.. i don't get why people like them so much. Both are so extremely fake and are just there to pamper their guest though they have regular attendance of A listers exactly because of that because they are willing parts of the big Hollywood machine. It's about 80% fake laughing and interest faking only broken up by the occasional funny bits such as the Hashtag segments with Fallon or some of his games (Lip Sync battle is awesome though and led to an entire show on its own).
I think Youtubing Late Night Talk shows is the best to do.. you get the best bits in a short time frame and don't have to slog through an entire hour watching fake laughing and the current movie/media star promoting his product (most of their stories are not even that interesting and funny).
Somewhere outside is Colbert.. while he follows the general late night formula he is not trying to be a clown and there for pure safe entertainment purposes. Some of his interviews are actually very critical (especially if he has politicians on or some other person of power), and some of his jokes can be pretty intellectual but he has a way with people few others do and he does it with such ease that he doesn't need to pretend his ass off to make a guest feel comfortable.
If a joke is mildly funny he'll chuckle instead of nearly falling out of his seat like Fallon but if it's really funny he'll also laugh for real. Colbert is a truly worthy successor to Letterman, bringing in his own style and i like him very much. However it launches into the stratosphere the few times he has Jon Stewart on.. his recent video of giving Stewart his chair and letting him do a Daily Show segment had me both in stitches and sad at the same time that Jon stopped doing that show.. once can only imagine what he would have done with the current presidential election.