Letterman is awesome, but in his younger days he was funnier. Like when Shirley McLaine stormed off the set or the time Cher called him a @%*$ing $*#hole.
Leno is just trashy humor, bad jokes but without the self-effacing comedy that Letterman brings. Leno just still seems that tiny bit insecure as to whether anyone's actually buying the idea he's supposed to be funny. There's no improv on the Tonight Show, it just feels so scripted.
Kilborn flirts incessantly with the female guests, and has a different comedic form than Letterman. Personally I can't stand the guy.
Letterman is where it's at