The Good, The Bad and The Ugly has a classic end if I remember right.. I haven't seen it in a while, i'll have to dig it out again!
I loved the end of Before Sunset - it was so sudden but so good. His smile when he says "i know" is just wow. and the end of American Beauty is great - with that whole long speech "i know you probabably have no idea what i'm talking about, but don't worry. You will someday". I got chills.
Butch n Sundance, Dr. Strangelove, and esp. the Sixth Sense, were all awesome endings. I have to say, though, that the ending of 'The Usual Suspects' was my favorite. I laughed my head off.