Anthony Hopkins will always be Hannibal Lector.
Yes, Everytime I see him I think that’s Darin Peter Oswald. 
Jon Favreau will always be the dweeb in Swingers
I can’t believe I’m the first person to bring up Jaleel White. I’ve seen him in other work, but I autmatically think “Steve Urkel” whenever I see him. Alot of people do (much to his displeasure).
Mark Hamill = Luke Skywalker.
This is a minor variation on the theme in the OP (as I interpret it) but when these actors are mentioned I usually associate them with these roles. Not necessarily their best but to me most closely associated with their style:
Gregory Peck – Atticus Finch
Humphrey Bogart – Rick Blaine
Dustin Hoffman – Babe (Marathon Man)
Gene Hackman – Popeye Doyle
Clint Eastwood – Dirty Harry (and Josey Wales)
Alan Ladd – Shane
Jack Palance – Wilson
Steve McQueen – Frank Bullitt
Charlton Heston – Moses
Paul Newman – Luke
Robert DeNiro – Travis Bickle
Salma Hayek – Santanico Pandemonium
Hedy Lamarr – Delilah
Natalie Wood – Judy
Glenn Close – Alex Forrest
The very first thing I ever saw Dame Judi Dench in was “As Time Goes By”, thanks to the local PBS station. And ever forward, despite her numerous and very well-acted other roles, she will always be Jean Pargetter Hardcastle to me.
Ron Howard - Opie Taylor
Christopher Reeves, too.
Ted Danson has done a ton of roles, but he’ll always be Sammy to me.
Whatshername will always be Jane Tennyson.
Tom Selleck will always be Magnum, PI.
Carol O’connor will always be Archie Bunker.
This for me, too. He was Chief Gillespie on “In The Heat Of The Night” for almost as long as he was Archie, but he’s ALWAYS going to be Archie to me.
Edward G Robinson said people always thought of him as a gangster yet he figured he played one only five times.
George C Scott was, despite not having a squeaky voice, George S Patton
No matter what role I’ll see him in from now on, Hugh Laurie will always be Doctor Gregory House.
William Daniels–John Adams
Art Carney–Ed Norton
Michael Landon–Pa Ingalls
Dick Van Dyke–Rob Petrie
Henry Winkler–Arthur Fonzerelli
Bea Arthur–Maude
And then there’s Bob Newhart, who, no matter what he is named, is Bob Newhart
Not a chance. Whenever that dude finishes a sentence, I hear a comma, followed by a condescending “Michael.”
Now that’s interesting, because I always think of him as the asshole heart surgeon on St. Elsewhere.
I always see him as Bertie Wooster.
No, no, no! William Daniels will forever be Mr. Feeney!
William Daniels will always be associated with the William Daniels role.
Saw him last night in Two For The Road, but remember him very distinctly as Benjamin Braddock’s father in The Graduate and the first role I remember him in A Thousand Clowns.