Peter O’Toole as Lawrence of Arabia
Kate Hepburn as Queen Eleanor in Lion in Winter
Denero as Jake Lamotta
Paul Newman in Cool Hand Luke
Tom Baker as Doctor Who. He wasn’t the first, but he made the role his own.
Charles Laughton as Capt. Bligh
Burt Lancaster as Elmer Gantry
Hoffman as Tootsie and Ratso Rizzo
Paul Newman as Hud
Sidney Poitier in Guess Who’s…
Ray Liotta as Henry Hill in Goodfellas
Rodney Dangerfield as obnoxious developer in Caddyshack
Brad Pitt as Pikey in Snatch
John Belushi as Bluto in Animal House
Robert DeNiro as Max Cady in Cape Fear
John Cleese as Basil Fawlty in Fawlty Towers (TV)
Nicholas Cage as H.I. McDunnah in Raising Arizona
Dustin Hoffman as Jack Crabb in Little Big Man
Heh. When I first read this I thought of Gandalf in the Mines of Moria doing a little dance, right out of The Treasure of Sierra Madre.
Then I remembered that that was Walter Huston.
:o
For me, Richard Harris is King Arthur.
Not that I haven’t enjoyed seeing other actors play him (particularly Graham Chapman), but those are actors playing a role. When I picture the “real” Arthur, it’s Richard Harris.
Or rather, he was Arthur. 
Errol Flynn as Robin Hood
Bette Davis as Queen Elizabeth the First
Cagney in anything, ditto Bogart.
Lauren Bacall as Marie in “To Have and Have Not”
Boris Karloff as Frankenstein’s Monster
Cary Grant as Mortimer Brewster in “Arsenic and Old Lace”
Katherine Hepburn as Tracy in “The Philadelphia Story”
Leslie Howard as the poet/tramp in “The Petrified Forest”
They stopped making movies around 1945, didn’t they?
No?
Could have fooled me!
Yul Brynner as the King
Hugh Jackman as Wolverine
Alan Rickman as Snape
Peter O’Toole as Henry II in both Becket and The Lion in Winter. Hell, the entire cast of The Lion in Winter especially, as eenerms already mentioned, Katherine Hepburn.
Peter Firth as Alan Strang in Equus
Tom Hulce as Wolfgang Amadeus Mozart and F. Murray Abraham as Antonio Saleri in Amadeus.
Gary Oldman as Ludwig van Beethoven in Immortal Beloved
Nicole Kidman as Isabelle Archer in The Portrait of a Lady.
Jimmy Stewart as George Bailey in ‘It’s a Wonderful Life’.
Cary Grant as Mortimer Brewster in ‘Arsenic and Old Lace’.
Rowan Atkinson as Edmund Blackadder, Tony Robinson as Baldrick, Hugh Laurie as George and Stephen Fry as Melchett. Those four gentlemen were born to play those respective roles. I cannot imagine anyone else doing justice playing those characters.
I realize he’s already got one in this thread, but I think Connery owns a second one besides Bond:
Retired LAPD Special Services Officer John Connor in Rising Sun.
Of course, it could be because of the fact that when Michael Chrichton wrote the novel, he specifically had Connery in mind as the character.
[geek]
I think Patrick Stewart could have played Gandalf.
“You must return the ring. Make it so.”
Also, I think a bloated William Shatner would have been great in tights as Superman.
“… fasterthan … aspeedingbullet … ableto … leaptallbuildings… ina … singlebound”
[/geek]
Sean Connery is James Bond. The others are actors playing James Bond.
I agree with brianmelendez, but also want to add Michael Richards for his role as Kramer on Seinfeld .
I vote for all of Spinal Tap. In fact, every Christopher Guest film is perfectly cast, though it helps that Waiting for Guffman and Best in Show have largely the same cast. Other than Guest, Michael McKean, Parker Posey, Bob Balaban, Eugene Levy, and Catherine O’Hara in particular own their roles. Then again, Bob Balaban owns his roles even when he does commercials.
Caroll O’Connor as Archie Bunker
Tom Hanks as Forrest Gump
Cleavon Little as Bart, the Sheriff of Rock Ridge in Blazing Saddles.
I’ve thought about it & decided, for me, that Elijah Wood, Sean Astin, and Viggo Mortensen definitely own their roles as Frodo, Sam, and Aragorn. (The LOTR movies are extremely well cast. Casting can make or break a movie, can’t it.)
Older movie role no one’s noted yet: Rosalind Russell as Auntie Mame.