Funniest Letterman Episode?
 

Personally my favorite episode is the one from last fall where Jack Hanna is on. It involves a giant lizard trying to eat Dave, Jack Hanna with some weird ferret down his shirt, and a lot of inane hilarious mumbling between the two men.

Monologue, musical guest, Paul trying to be funny....none of this matters. The weird synergy between Dave and Jack makes the whole show.