I can't get enough of the moments of film when the hero meets the villian, usually thinking they are not the villain. My two favorite scenes of this type:

Die Hard: I love the brief exchange between Alan Rickman and Bruce Willis when they first actually meet. I love the scene because John McLane gives him a gun with no bullets. Their radio conversations are much better though.

Heat. The diner scene with Al Picano and Robert Di Nero is classic. This scene...well besides the badass shootout in the streets made this movie. But of course Hollywood fucks up yet again and puts these two together in a piece of shit.