I think he is underrated by the masses but the people who follow his work rate him very, very highly. Same goes for Christian Bale. Both have loyal fanbases and receive almost worship from their fans.
I agree with Naomi Watts (she is talented, beautiful, Oscar-nominated, was the heroine in King Kong and yet still so many people either haven't heard of her, or confuse her with Nicole Kidman!)
Also like to add Ralph Fiennes name to the list.