What are some movies where the lead actors have excellent, on-screen chemistry?

I am just watching the end of The Taking of Pelham 123 and I gotta say, I think that John Travolta and Denzel Washington should make ten more movies together. Those two have the most amazing on-screen chemistry and that made the movie that much more interesting and enjoyable.

Actors get awards for portraying characters really well on-screen but they never get awards for playing off another actor really well. Can you tell me some movies where you think the lead actors just acted really well together?