Yes, you are. Best performance ever was Robert Duvall in Lonesome Dove, and a miniseries is better than a movie.
Lazy, fun-loving guy; competent cowboy; loudmouth tough guy (in the bar); quiet deadly tough guy; bragging womanizer; tender lover; to name a few facets of the role.
For just totally stepping outside their usual comfortable oeuvre, I’ll nominate Charlize Therron for Monster. Totally unexpected and totally nailed. For overall body of work currently, I’ll nominate Phillip Seymour Hoffman and Ed Norton.
Honorable mention to André Tchaikowsky, as an utterly convincing Yorick.
Anthony Hopkins in The Silence of the Lambs
James Dean in East of Eden. I have never seen anything to match it, before or since.
There’s a scene with just him and his girl, just outside a house. As the dialog progresses he wanders away from camera and disappears behind a tree! The actress, if I’m any judge, was not expecting this and it adds immensely to her sense of bewilderment at this man she’s with.
As he keeps delivering lines from the darkness your attention is utterly riveted on that tree, almost begging for a glimpse of him to restore some sense of cinematic normalcy. Dean extends you no such mercy, and demands that you as a viewer have to take it on his terms.
Actually I thought Robert De Nero in* Awakenings *was just about the best performance ever by an actor.
Of course you know this means war. 
I’m gonna say for men: Gary Oldman … he vanishes into his roles and you almost don’t realize it’s him when you see him in things.
For women, I know it’s cliché but it’s true: Meryl Streep. She becomes her roles unlike any other actress I’ve seen.
I’ve heard actors speak of Olivier in hushed tones, so I would give him the nod.
Streep is incredible in the way she inhabits roles.
For my money, Daniel Day Lewis is the finest working actor today.
Gene Wilder in Young Frankenstein.
IMHO: Morgan Freeman is a star, not just an actor. As a star, he’s always playing himself in some form or other. I’d nominate Katherine Hepburn for “best actor ever” but she has the same problem, she’s always Kate Hepburn.
Robert De Niro.
In the early part of his career, I kept taking note of especially outstanding performances in various films, and when I waited for the credits to roll, they all turned out to be De Niro. I really didn’t know it was the same actor from film to film. Look at him from Mean Streets to Taxi Driver to Raging Bull, just to pick out a few.
For the scope of the OP, I think “best ever” has to be based on career quality, not just one role in one good performance or character (e.g. justifying ‘best ever’ on Godfather).
My short list…
Men: Gary Oldman
Women: Jodie Foster