I always loved the expressions on Christopher Walken's face during the second Russian Roulette scene in The Deer Hunter; he runs through a chilling array of dead still, jittery nervousness, and scariest of all, a sort of detached bemusement like he's about to let loose a chuckle as the scene is unfolding all around him.
Perhaps my very favorite performance of all time is Anthony Hopkins in The Remains of the Day, just from start to finish; he communicates a range of emotions through this veil of detached servitude. I saw it six times, and never got tired of it. I can't tell you how pissed off I was that Tom Hanks stole Sir Anthony's Oscar for his two-note gimmick role in Forrest Gump. And I am not a Hanks hater, but that really made me angry, and you don't want to see dfk angry.
Meryl Streep in Sophie's Choice.
And I thought John C. Reilly was pretty phenomenal in every scene he had in Magnolia.
Way too many to mention, really, but that won't stop me trying.