The funniest scene I can think of right now...
Dear lord it still gets me every fucking time
The saddest one I can think of right now is this scene from Reign Over Me, the best acting by Adam Sandler to date (if you don't count Happy Gilmore.)
Skip to 2:40 for the actual monologue, but for the whole set up, watch the whole clip. If you haven't seen the movie, it's quite good, I recommend it.