I voted for Gary Oldman, as he's a brilliant actor, but the more I think about it, the more I realise he hasn't really given many performances that SHOULD have been Oscar nominated but were overlooked. He tends to either do non-showy roles that are undoubtedly strong but not really Oscar-friendly, or he's playing show-stealing supporting parts in the kind of big budget blockbusters that would never be under consideration for acting Oscards. I do think he should have gotten Best Supporting Actor nominations for his work in Leon and Murder in the First. But if he is to become a serious Oscar contender, he needs to start doing more leading roles. With the right role, and the right marketing, the Oscar will surely be his.
But speaking of Murder in the First, there's another great actor with a whole bunch of Oscar-worthy performances, who to my knowledge has never recieved a nomination: Kevin Bacon. I think he gave easily the best performance of all three leads in Mystic River, but was the only one not to go home with an Oscar.
As for the people on the shortlist:
BILL MURRAY
Not really had an Oscar friendly career, with much of his "serious" work outside of Lost in Translation being good rather than great. But out of the Best Actor nominees for the 2004 Oscars, he was the most deserving. I'd still rank others on the shortlist ahead of him, though.
TOM CRUISE
Tom Cruise's biggest obstacle is that he's Tom Cruise. Because of Tom Cruise - The Brand, Tom Cruise - The Movie Star, Tom Cruise - The Eccentric Scientologist, the fact that he's actually a very talented actor when he wants to be is too often overlooked. He should have won the Best Supporting Actor Oscar for Magnolia, that performance was a tour de force, a standout in an incredible cast. But even in his younger days, Born on the Fourth of July was a great performance, and the only reason it didn't win is because it had the misfortune of falling on the same year as an even more harrowing, even more convincing performance of a crippled man, from Daniel Day Lewis in My Left Foot. I'm also of the controversial opinion that it is Tom Cruise who gives the superior performance in Rain Man, with a more thankless role. He seems to have given up on any Oscar hopes though, more content in doing action films lately.
JIM CARREY
Both The Truman Show and Eternal Sunshine of the Spotless Mind were great, Oscar-worthy performances, and it's terrible neither were even nominated. Though while when he's on, he's really on, his work can be a bit hit and miss.
EDWARD NORTON
I'm not as big a fan of his as others are, but American History X was incredible.
LEONARDO DI CAPRIO
Leo needs to be in some kind of disfiguring accident, or start getting old and really showing his age. Because it seems to me like the only thing holding him back is his pretty boy looks, and the way that makes people percieve him. His performance in The Aviator was better than Jamie Foxx's Oscar-winning performance that year as Ray Charles (though the not-even-nominated Paul Giamatti is the guy I think should have won the Oscar that year for Sideways), and if he'd been nominated for his work in The Departed rather than the lesser Blood Diamond, he might have been the one guy with a shot at upsetting Forest Whitaker for the 2007 Best Actor prize. He's a consistently great actor, and it's only a matter of him getting the right role before he gets the win.
PETER O TOOLE
A legendary actor, who should have gotten an Oscar way back when. But to give him one now would just be more a "thank you" Oscar for his whole career. And that's what an honorary Oscar is for - and he's got one of those...
VIGGO MORTENSEN
He's had the bad luck of giving his best performances in really competitive years - who could have hoped to upset the Daniel Day Lewis in There Will Be Blood juggernaught? Great actor, but I wouldn't quite rank him up amongst the very top of this list yet. He needs a few more truly great roles under his belt.
BRAD PITT
Like someone else said before, while he's a very talented actor, there haven't been that many roles of his that you can really think "he should have won the Oscar for that". In my opinion, his career-best role was in the little-seen The Assassination of Jesse James by the Coward Robert Ford. And even in that, he was surpassed by the excellent work of one Casey Affleck (who I think is well on his way to earning a place alongside these names in future years, if he doesn't win the next Best Actor Oscar for The Killer Inside Me...)
ROBERT DOWNEY JR.
One of my favorite actors, I love him. He's an incredible talent, full of charisma, and is consistently entertaining in whatever type of film he's doing. But like with Gary Oldman, there aren't that many roles he's been in where you'd immediately think "Oscar worthy". There's Chaplin, and he got the deserved nomination for that. I'd say Zodiac, but its April release prevented from ever touching on Oscar radar. I have an idea - somebody make a movie with Robert Downey Jr and Gary Oldman in the two leading roles, and release it at Oscar season.
CHRISTIAN BALE
He should have won it for American Psycho. Yes, he should have beaten Russell Crowe for Gladiator. Since then, he's given a number of strong performances, but again no obvious Oscar-baiters. His time will come in the future.
JOHNNY DEPP
Yeah, definitely a candidate I'd put near the top of the list. His work in Finding Neverland was also better than Jamie Foxx in Ray, he outacted Al Pacino of all people in Donnie Brasco, and I think Ed Wood was not only Tim Burton's masterpiece, but Johnny Depp's finest hour and he should have at least been nominated for it. I think many thought that Public Enemies could have been his shot at Oscar glory, but when the film actually came out, it turned out to be too evasive and abstract an experience to sway Academy voters. But still, it's only a matter of time.
ALBERT FINNEY
I haven't really seen enough of his work to comment, though he was great in Erin Brokovich and especially Miller's Crossing.
MATT DAMON
One of the most underrated actors out there. People raved (rightly so) about Leonardo Di Caprio, Jack Nicholson, even Mark Wahlberg in The Departed, but the true star of that movie, in my opinion, was Matt Damon. Excellent in The Talented Mr. Ripley. Compelling in Syriana. Bonkers in The Informant! And he's hilarious when he wants to be too, as seen with his obscene self-parody in Entourage. He's a dependable, versatile actor who, much like Leonardo Di Caprio, isn't given his due largely because of his good looks.