I agree with several of the best performances choices already. Certainly Ian McKellan in Amadeus, which was absolutely a brilliant tour de force the likes I've never seen. I also saw Mr. Robards in Hughie and he was magnificent. And Jennifer Holliday was incredible, too. But I'd add to the musical list - Tammy Grimes in Molly Brown, Joel Grey in Stop the World, I Want to Get Off, Robert Morse in Sugar, William Daniels in 1776. Dorothy Collins in Follies. For plays, I'd also add Hermione Gingold in Oh, Dad, Poor Dad, Mama's Hung You in the Closet and I'm Feelin' So Sad, Kendall Clark and Michaele Myers in Who's Afraid of Virginia Woolf (the matinee tour cast), Tammy Grimes and Brian Bedford in Private Lives, Anthony Hopkins in Equus (a brilliant turn), Mary Louise Parker in Prelude to a Kiss and Communicating Doors, and Lily Tomlin in The Search for Intelligent Signs of Life in the Universe.