Most of the time, it seems as if the Academy is playing catch up, as they will be doing this year if they give awards to Jeff Bridges and Sandra Bullock. There have been so many great actors throughout history and comparatively few awards to give. And every time the Academy gives an Oscar to some flavor-of-the-moment (Paul Lukas for Watch on the Rhine in 1943 for example), it takes more time to catch up. Usually they catch up with actors late in the game, and they usually only notice when the actor plays either 1) a real person, or 2) someone with a disease or malady of some kind. So when an actor actually wins for a truly great performance, it's something to celebrate. The following is my selection for the best choice in each of the four acting categories from the entire history of the Oscars. Let us know what yours are!

Best Actor:
Gene Hackman for The French Connection (1971)

My first thought here was Robert De Niro for Raging Bull, but you could make an equal or better argument for De Niro in Taxi Driver, and not to mention that it's one of those very common biopic awards. But I'm going with Hackman, because for once the Academy recognized a great actor in his early prime, and they awarded him for a very atypical movie. The French Connection is a superbly detailed cop/chase with some rich characterizations, and it may have been a heat-of-the-moment Oscar for a hit movie, but it has really stood the test of time and remained one of the best and most entertaining movies of the 1970s. Hackman is absolutely fearless in his role as Popeye Doyle (who can forget that name?), caring not the least bit for likeability or redemption.
Other nominations: Bonnie and Clyde(1967), I Never Sang for My Father (1970), Mississippi Burning (1988)
Other wins: Unforgiven (1992).