To me a good monologue always explains where the character is at that moment and how he got there. Most of the classic ones I have seen describe a character's former way of thinking and then what changed that way of thinking. Passionate and emotional monologues I think work best -- characters trying to convince other characters (and the audience) of their point of view.

It's amazing how everyone forgets Other People's Money, I honestly put it up there with Wall Street.