I'd say Christian Bale for underrated. When people saw him in Batman, you would think, meh they gave him a very boring actor to act for, pretty straight forward to do, and he got all these plaudits. Then you watch Fighter and The Prestige and think, WOW. Pretty awesome.
Also think Collin Ferrel is underrated. Or is he? Meh I think hes pretty good actor too.
Overrated- Will Ferrel I agree. Hes just not funny.