This year, 2009, Sean Penn is nominated again with a Best Actor Academy Award for Milk.
His first Oscar was given for his performance in the 2003 film Mystic River. But everyone keeps saying that he should've gotten an Oscar two years earlier for I Am Sam (but as Robert Downey, Jr. said in Tropic Thunder, Sean Penn went full retard for the role, and came home empty-handed).
But the truth is, he should've gotten an Oscar way back in 1995, for this role:

Penn's portrayal as a death row convict is so convincing that I was actually both afraid and awestruck at the same time. Afraid because I realized that somewhere in the far reaches of the United States, there still exist humans like Poncelet, cold-blooded evil racists who could kill someone without even batting an eyelash. And awestruck because even stone-cold killers can break down in tears when faced with death.
The Academy of Motion Picture Arts and Sciences is not an infallible institution. Their choices are not divine in nature. They are merely opinions of the majority of them. Watch the Oscar nominees for Best Actor in 1995 again, and I guarantee you'll be shaking your head and wondering how in the world Nicolas Cage went home with the Oscar for Leaving Las Vegas when Sean Penn was clearly the best actor for that year.