Who's an actor who nailed a role so hard that nobody else will ever be able to live up to it?

Mike Myers as both Wayne and Austin Powers. He has a way of doing corny characters with soul, and that's quite rare.

Bruce Campbell as Ash Williams in the 'Evil Dead' movies.
I fucking dare you to try to remake that with another actor.

Matthew Mcconaughey as Rust in True Detective S1. I'm desperately longing for a show that can live up to S1 with a character similar or better than Rust
- Every character in that season was perfect. Even the very minor roles played by unknowns were all knocked right out of the park. The stars really aligned for that first season, and I am very much okay with that being a rare thing.

Some great runner-ups mentioned here. Of course the answer is Dr. Frank-N-Furter, who also had one of the best character entrances ever.

Anthony Hopkins as Hannibal the Cannibal
- I thought the same,then i watched Mads Mikkelson in the tv series Hannibal and think he played the role even better.

Heath Ledger as the Joker.
Robert Downey Jr. as Iron Man.
- I think Heath Ledger is the best possible version of that type of joker. I think Mark Hamill was amazing too though.
  RDJ just is iron man though. Similar life paths and everything.

Hugh Jackman as Wolverine. With the sole exception of his height, (comic book Wolverine is very short, and Jackman is not) it’s like the character walked out of the comics and onto the movie screen.
- The like, 2 seconds in Deadpool where they had Cavill as wolverine made me wonder if he could legitimately do it
- Hard disagree. Hugh is too tall and too pretty. He's done a fine job with the character but I am extremely excited for a new short-king wolverine. It's time to move on.

Alan Rickman as Severus Snape
- AND as the Sheriff of Nottingham!
  
  Why a spoon cousin, why not an axe? It's dull you twit.
  I always remembered that bit, I rarely heard anyone but my dad call someone a twit.

Bryan Cranston as Walter White
- Bryan Cranston on It's always sunny. The man's got range!

David Suchet as Hercule Poirot.
- One of the things about his performance is that the role has also been played by more than a dozen other actors, including Kenneth Branagh, Peter Ustinov, Albert Finney, John Malkovich an Ian Holm. Yet he made the character indelibly his own.

I used to think no one but Jim Carrey could do Ace Ventura, but I've seen plenty of impersonations of the character now that Carrey actually wouldn't be needed.
I used to think nobody could replace Mark Hamil's Joker voice until the devs of the Arkham games debuted his replacement for one of them and he fucking nails it to the point if I didn't know he was replaced, I would just assume it was still Hamil.
I can no longer hold the idea that an actor is irreplaceable simply because a good actor can imitate other actors.