I could do this all day, here are a few:

Robert Mitchum as Phillip Marlowe in any Chandler adaptation circa late 40's to mid 1950's directed by either Nicholas Ray or Robert Aldrich.

Kubrick's Napoleon with Jack. I don't know why the idea of Nicholson as Napoleon just seemed so insanely right...kinda...or heck I'd see this whoever was in the lead.

And one of my own...

Let's get a coffee (AKA The Ultimate Guy Movie, AKA The Man Who Would Be King II): Michael Caine and Sean Connery get together and have a cup of coffee. Then they run into Peter O'Toole. Chaos ensues as they hit the town. Explosions, fights, murders, brawls, car chases, a bit of gratuitous sex and everything in between. Cameos by: David Bowie, Pete Townshend, Albert Finney, Kevin Spacey, Robert DeNiro, Harvey Keitel, Al Pacino, Chow Yun-fat, Roger Moore and many more.