Bill Paxton will forever be Hudson from Aliens…
Keanu Reeves as Bill in “Bill and Ted’s Excellent Adventure”.
He’ll always be Devil from Justified to me.
And I can’t believe I’m the first person to mention Tony Soprano, er, I mean James Gandolfini.
On that note: Marlon Brando IS Don Corleone.
Devil, fer sure. Too stupid to live. “Kenny” didn’t do or say much one way or another to invest me in the character.
But James Gandolfini had six seasons to “own” Tony Soprano. By contrast, Joseph Siravo, with only spot appearances in 5 episodes, is now and forever Johnny Boy Soprano (Tony’s father).
No, he was Ted “Theodore” Logan. Alex Winter was William S. Preston, Esquire. (Bill.)
Two for me:
Although he was Capt. Archer on Enterprise and was in Men of a Certain Age and now NCIS New Orleans, Scott Bakula will always be Dr. Samuel Beckett from Quantum Leap for me.
And although he had numerous film roles, including in Bridge on the River Kwai, Sir Alec Guinness will forever be Obi-Wan Kenobi.
I have a really obscure one. On Law and Order, S. Epatha Merkerson played Lt. Van Buren for YEARS. But before they introduced that character, she played a crackhead on the same show a season or two before. For all the time she was Van Buren, I was always in awe of the transformation she made from that crackhead, but I still saw a little of it in her.
One of the SVU DAs, Diane Neal I believe, played a vic several seasons earlier.
Actually, she played a perp, a female rapist and murderer. Very bad girl.
To my mind, Chris Cooper is and shall always be Bob Cody.
And let me tell you, that made watching the 2011 Muppets movie weird.
Clancy Brown will always be the Kurgan.
Garret Dillahunt will always be two characters in my mind (both from Deadwood): Jack McCall and Francis Wolcott. And much like the description of (the excellent) S. Epatha Merkerson’s remarkable transformation from crack whore to police captain above, when many people tuned into the second season of Deadwood, they hadn’t the slightest idea that the man playing the refined, handsome, and utterly insane “Mr. W.” was the same actor who played the filthy drunk Jack McCall in the early episodes of the show’s first season.
Sam Waterson IS Jack McCoy.
Laurence Fishburne will always be Morpheus. I don’t care what role he’s playing, I know he has those pills in his pocket.
I recently realized that the woman who has to put the lotion on her skin has a part in Ray Donovan. Now it’s hard to think of anything else when she’s in a scene.
Ted, but yeah that’s what I came to say.
Not because the role was so iconic. Just because it appears to have been the only role that he could really be convincing at.
Sean Connery as Bond. James Bond. 
Easiest way to remember this is to recall the line “Shut up, Ted!”
Not only that, Robert DeNiro won an Oscar for playing Don Corleone and Marlon Brando is STILL Don Corleone.
Nit Pick
The episode was Mushrooms. Her character was a hard working cleaning lady, not a crackhead, whose 12 yr old son is shot and her baby is killed when the hit man (a young boy) went to the wrong address because he could not read.
Don’t know if this counts, but I can’t watch anything with somebody from Whose Line Is It Anyway, especially Wayne Brady and Colin Mochrie (Ryan Stiles I saw in several other things, particularly The Drew Carey Show, so it doesn’t affect him much), without thinking, “That’s the guy from Whose Line!”