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).

Best Actress:
Janet Gaynor for 7th Heaven, Street Angel, and Sunrise (1927)

It may seem a little precious to say that the very first Best Actress winner was the best -- and I almost went with Holly Hunter in The Piano -- but this Oscar went to three great performances in three great movies for the price of one! Gaynor's career stretched about ten years, not counting some bit parts before 1927 and a few TV appearances after 1938. With her big eyes and doll's face, Gaynor usually played sweet characters, often put-upon by others. However, in these three grand, visually striking artistic masterpieces, she perfectly fits the passionate, poetic material by the great directors F.W. Murnau (Sunrise) and Frank Borzage (7th Heaven and Street Angel). She emits tragedy with the tiniest gestures; she anchors the films. She's the doorway for the audience.
Other nominations: Nothing Sacred (1937)

Best Supporting Actor:
Heath Ledger for The Dark Knight (2008)

This is the toughest category, because there's an awful lot of quality work here. It seems largely to be the most adventurous category in the history of the Oscars. I had several top contenders, including Edmund Gwenn, Walter Huston, Walter Matthau, Robert De Niro, Michael Caine and Kevin Kline, but I choose Ledger for several reasons. For years, I was firmly in the anti-Ledger camp. I thought he was bland and humorless. He shut me up and turned me around like nobody's business. (I have even revisited and appreciated some of his earlier roles.) This is arguably Ledger's best work, an iconic performance that has linked his name with the Joker for all time (forget anybody else who ever played him or ever will play him). It's one of the great movie villains; he's insane, but he's so smart that you can see his reasoning behind his madness. I have become a fan, and I wish now that we could have seen much more from Ledger, who had clearly reached a major plateau.
Other nominations: Brokeback Mountain (2005)

Best Supporting Actress:
Marisa Tomei for My Cousin Vinny (1992)

I also adore Gloria Grahame, Mary Steenburgen and Penelope Cruz, but Marisa Tomei's award was another one that came early in a strong career, and for a comedy. I, along with most other moviegoers, have never forgotten the scene with her demonstrating her biological clock. There was a small controversy following her award: it was suggested that presenter Jack Palance could not properly read the contents of the envelope he was given, so he pulled Tomei's name out of thin air. This theory has never been proven, but what has been proven is that Tomei is no fluke. She has been nominated twice more since, and has also appeared in a dazzling variety of scene-stealing, funny and/or soulful supporting roles over her 20-year career (although, despite her beauty and talent, she has never really caught on as a leading lady). You know an actor is good if they can be great in something like What Women Want.
Other nominations: In the Bedroom (2001), The Wrestler (2008)