Conan, without a doubt. The Walker Texas Ranger lever is the most amazing creation of all time. I rate it even above the polio vaccine. I don't even bother watching any of the others. John Stewart is great, as well.
I used to sometimes watch Conan, but I got tired of it after a while...I still occaisonally watch it though.
I don't know if this is considered a late night show since I usually watch the rerun of it at 4pm so I really don't know what time it airs...but I love the Daily Show with Jon Stewart. Usually I find that with shows on every day they might only have a few good jokes where I actually laugh, but with The Daily Show they always seem to be very fresh and they make me laugh out loud (L.O.L) with almost every joke.