I'm with you, Lorenzo
I'll miss Letterman, his show aged as he did - and I did. The new guys are for generations younger than me. Not that I don't find a lot of what they say and do funny. I'm not that out of touch.
Unlike some, I did think Letterman was often very funny and though I cut my teeth on Carson and love him, Letterman is my favorite-by the way, he was Johnny's favorite too. I'm sure you all know/remember that Dave was Johnny's choice to take over the Tonight Show after he left. They continued to stay in touch with each other, remaining close until Johnny's passing.
Comedy is so subjective, I've always said that all comedy is stupid and who is funny to you depends on what type of stupid makes you laugh. Leno never did it for me either as a stand-up comedian or on the Tonight Show where I thought he was at his worst. Yet, like Budweiser, more people bought him so I'm not so sure you really want the "most viewed" moniker, if you've got integrity as a comedian.
I think Colbert can be very funny, as can most everyone mentioned, it's doing it night after night that makes these late night spots so tough. Fallon's got his good points and I think will continue to do well, appealing to younger viewers. I actually like him, I just wish everyone of his guests wasn't his "favorite _ _ _ _ _ _ _ !" (you fill in the blank).
I agree Fallon's musical skits and impressions are the best which makes him unique as he can really make musical guests more relaxed and more interesting as they don't feel so out of their element sitting on a couch trying to answer questions they can't relate to or don't capture anything relevant about them or their music. It's cool to see musical guests having fun and getting involved with the show instead of looking uncomfortable.
Apparently I was in the minority in liking Craig Ferguson but haven't had a chance to see his replacement yet but I hear he's funny too. Anyway, that's a little of what I think, not that it matters.