Which actor/actress are you unable to dissociate from one of his/her role?

I never watched an X-men, nor the old Dune, so for me Patrick Stewart is Jean-Luc Picard.
- Agree! When I see him in other things I'm like "oh hai Picard"
  
  Voice of Avery Bullock in American dad. In Picard show, is his age caught up to him and he looks super disinterested and tired

Sarah Michelle Gellar will always be Buffy Summers to me.
- I somehow missed Buffy growing up, so my first exposure was of her as Daphne in the Scooby Doo movie.
  I think now that I've watched a lot of Buffy, those two balance out for me.

Roger Moore as Bond - he wasn't the best Bond, just that he played that character for like 30 years, starting with The Saint in the 60's.
Once you see him there, it's hard to see him as anything else.

William Shatner and Leonard Nimoy as Kirk and Spock in the early Startrek films and series.
- Same for Brent Spiner as Data.

Since 1995, I’ve always associated Ian McKellan with his iconic portrayal of Richard III.
Which colored my impression of Lord of the Rings a bit.
- Now is the winter of our discontent, but that is not for us to decide....
  
  My kingdom for the lord of all horses!

Danny Trejo is of course Machete (or Uncle Machete to the Spy Kids) - even if he does occasionally rangle Rancors or other roles.
(But only when I can see him. I had no idea he had that many voice roles until looking him up just now)

Jaleel White is forever Steve Urklel.
Alfonso Ribeiro is Carlton.
- Alfonso Ribeiro will always be FADDER!
  SPEAK UP, WE CAN'T HEAR YOU.

Mac from IASIP. Saw his picture with his son and immediately got thinking 'place away this psycho from a child'
- His plastic surgery made him look all strange, and he's pretty pretentious as a person too, and addicted to the Hollywood lifestyle. If you watched the interviews he given over the years you will know. Associating himself with Ryan Reynolds because he wanted the ted lasso fandom, and a famous retire American football star.
  Even his fans on the isaip subreddit was thrown off by Rob and Kaitlyn appearances
  
  All I could find was that he lifted his butt. Honestly, it's the most Mac choice and I can believe in whatever I want at any given moment based on the argument I'm trying to make

For me, it's David Tennant as The Doctor. It was very strange seeing him in Good Omens.

Daniel Radcliffe.
After seeing Swiss Army Man, I can never not see him as a useful corpse. He nailed that role.

Idris Elba and Dominic West will always be Stringer Bell and Jimmy McNulty for me.
- Idris Elba
  Nah. He can only be Luthor.

Joe Rogan is Joe Garrelli in News Radio
- I like to remember him in that role as it was when I liked him as a person and a professional.

Kind of obscure, but Brits of a certain age will always think of Alan Dale as "Jim from Neighbours". Another one being Guy Pearce as "Mike from Neighbours". Pearce has had a slightly bigger career than Dale maybe, but you've almost certainly seen them both in popular movies and TV shows.
- Excellent answer, particularly Alan Robinson/Jim Dale. Or whatever :-)

Young Guns will forever in my head have Jack Bauer in a leading role.
- You mean Lost Boys vampire gang leader David?

Peter Lustig. But he also played himself and had full power over his show.
Martin Freeman as Bilbo
Til Schweiger as his arrogant asshole self

Machlenny, Glenn, Kaitlyn from isaip, they just aren't good outside of the show. Also rob is pretentious as fuck. Typecasting is pretty much a curse for people like these

Claire Foy as Lady Percy in the Upstairs, Downstairs reboot. Seeing her play the queen just felt so wrong, lol.