I'm surprised no one has mentioned River Phoenix and Keanu Reeves in "My Own Private Idaho" [unless of course I've missed it in my quick scanning of the previous entries...]
It's reported they had a real good time and lots of fun while preparatory work for and while filming Idaho. I think you can see this on screen. They really matched up.
Keanu may not be the best actor. Yet in MOPI, paired with River, he was really not bad. But it was River who carried the movie.
I also think Keau matched up good whith Sandra Bullock, but I think this one was already mentioned.