powerhouse performances of all time

What is the most gripping, convincing performance you ever saw? so powerful it was so damn realistic, you thought Hitler was back alive (Bruno Ganz, Der Untergang) ?

Yes, Bruno Ganz was excellent in that film.
For me its, Peter O'Toole in Lawrence of Arabia. It's a must see, as far as I am concerned.

That film knocked me out in 1962! I've seen it several times since, and it's still one of my favorites. There had never been any film of it's magnitude before --and little since-- all photographed in glorious Super Panavision by Freddie A. Young.

O'Toole should probably have won the Oscar. Instead it went to Gregory Peck for To Kill a Mockingbird. He was good, but I always felt that Peck won because of the character he played rather than his acting. O'Toole acted the whole gamut as Lawrence. I just looked it up. He was nominated 9 times, but never won. Something wrong there...

Juliette Binoche, Three Colours Blue

When I first saw Slingblade (1996), I thought Billy Bob Thornton was a mentally challenged actor.

MMMMhmm!

Just off the top of my head when I think of the adjective "powerhouse":

Marlon Brando, A Streetcar Named Desire

Elizabeth Taylor, Who's Afraid of Virginia Woolf?

Robert De Niro, Taxi Driver

James Dean, Rebel Without a Cause

Daniel Day Lewis There Will Be Blood

Ellen Burstyn Requiem for a Dream

Paul Newman The Verdict

Andy Griffith A Face in the Crowd

Jack Nicholson One Flew Over the Cuckoo's Nest

Jack Lemmon The China Syndrome

James Dean East of Eden

Geraldine Page Interiors

Gene Hackman The Conversation

Paul Newman Cat on a Hot Tin Roof

Al Pacino Dog Day Afternoon

Jack Lemmon Missing

Piper Laurie Carrie

Mary Tyler Moore Ordinary People

Tom Cruise Magnolia

Sissy Spacek In the Bedroom

Ellen Burstyn The Exorcist

Jack Lemmon Glengarry Glen Ross

Meryl Streep A Cry in the Dark

Jack Nicholson Five Easy Pieces

Dustin Hoffman Midnight Cowboy

Meryl Streep Sophie's Choice

Definitely Michelle Pfeiffer as Selina in Batman Returns:

It probably hasn't been mentioned yet because it's obvious, but Orson Welles in Citizen Kane. Consider how convincingly he transitions from a young man to an old one and that he was only 25-26 while filming it, no less.