What do you consider some of the all time greatest performances by an actor in movie/show, and explain what about them you found so great?

Two great actors at their best:

I've recently watched The Father with Anthony Hopkins and Olivia Colman. The story follows a father suffering from dementia and her daughter trying to care for him the best she can. The film is presented from the point of view of the father so the viewer is not sure what is real of not. Anthony Hopkins' performance is breathtaking, he goes from cruel and mean, to absolutely charming, to confused, scared and back around again. It's not a easy film to watch because it breaks your heart to see the ravages of dementia presented that way but the performances are well worth it.

Kathy Bates in Misery is just phenomenal. She is scary as hell in that one.

People shit on method acting, but for my money DDLewis is the goat. Every single one of his perfomances is just brilliant.

De Niro in Raging Bull, Brando in On the Waterfront (and Godfather) also come to mind

Yeah, it's easily one of the best performances I've ever seen. Stunning work by Hopkins, and great directing as well, the sense of confusion makes it even better.

I'm gonna show my inner fanboy a bit here, but Sean Bean has been giving incredible performances his whole career.
He sure does a lot of stinkers, but whenever finds the right crew, he delivers my all time favourite performances.

Boromir - This is Oscar level. Everything about LotR was amazing, but Bean stood out to me from the greatest ensemble I think I've ever seen.
Boromir's struggle was so human and relatable; wanting to do right by his people, but falling to temptation and then of course his redemption. This is the role that made me a Bean fan for life. My mind exploded as a kid when I realized he also played my favourite Bond villain in my favourite Bond film.

Everyone is excited for Daniel Day-Lewis' return to acting, but I'm excited to see Sean Bean show he stands among the greats.
I'm happy he has gotten some well deserved awards recognition through his television career post Game of Thrones.
These are among my favourites too:
Accused (2012) - International Emmy
Broken (2017) - BAFTA
Time (2021) - BAFTA

Almost all the lead actors/actresses in the original Godfather.

And, if you've read the book, almost to a T the way King wrote her.

Dustin Hoffman in Rain Man. He deserved the Oscar back in 1988.