The usual British suspects: Alec Guinness, John Gielgud, Ralph Richardson, Cyril Cusack, Maggie Smith, Peter Sellers, Peter O'Toole, Brenda Blethyn. I think Olivier is generally overrated, and quite bad when he's bad (Othello).

Among Americans: Streep (though that's an obvious choice) and Robert Duvall ("Tender Mercies" in my biased opinion contains the best performance by anyone).