Most Funniest - Conan (w/out a doubt in my mind)
Best Guests - Leno
Best Interview - Letterman
Never been a Leno guy...just dont think he's funny, and it annoys me how he laughs at each of his jokes (even if they dont land). Letterman & Conan know to make fun of themselves when a joke doesnt make the crowd laugh. Leno seems to grin like a schoolgirl regardless.
If I had to pick one of the shows mentioned it would be Letterman by a pretty large margin. But, truth be told, I'm much more likely to listen to the radio late at night.
None of the above. I know you asked for network television, but I have to mention that for those that can get the Comedy Channel, I encourage you to check out Carlos Menecia on The Mind Of Menecia every Wednesday night at 10:30 p.m. for his weekly half hour show. He was on Leno and even the host couldn't stop laughing.