Gotta throw Brad Pitt in there. Hold on let me explain...... granted he takes on a lot of easy roles that don't take much adaptation, but he absolutely killed it in some very tough roles to play. Some of the guys already listed in here couldn't do what he did in Snatch, Inglorious Basterds, Troy, Benjamin Button, etc. the man gets regarded as a sex symbol for women and people seem to forget he's an incredible actor.
Also gotta go Day-Lewis. (needs no explanation)
And Pacino. (De Niro is a close one to Pacino but I feel like he rarely takes on difficult roles. Pacino did Scarface. To perfection)
There's my 3.
Honorable mentions: Tom Hanks, DiCaprio, Nick Nolte