Overrated:
George Clooney
Brad Pitt
Tom Hanks
Underrated:
Steve Buscemi (my favourite actor, check out Trees Lounge)
David Thewliss (you might recognize him from Harry Potter, but you must see him in Naked)
John Turturro (Do The Right Thing and Barton Fink immediately come to mind.)
David Strathairn (Brilliant in Good Night and Good Luck, also excellent in Limbo)
However, it really depends how we define over/underrated, because guys like Turturro and Buscemi are certainly held in the highest regard among cinema buffs, but they tend not to play the stereotypical leading Hollywood roles (although Buscemi has provided amusing supporting turns in otherwise forgettable Hollywood fare, such as The Rock and The Wedding Singer.)