If you were paying attention and betting against Ray Milland for 1945's Best Actor Oscar, you deserved to lose your money.
The versatile Canadian actor had done light comedy, musicals, horror, spy thrillers - and that was just his 1944 slate! With Billy Wilder's The Lost Weekend, he took on the role of a writer who spends his days and nights getting boozed up, and when he isn't, he's scheming how to score the next drink. Not exactly the most commercial of plotlines, Paramount had insisted Wilder cast the reliable Milland to at least guarantee some butts in seats. From its first screenings in London, the seats were filled and the critical praise effusive; by the time it reached the United States, trade columns were already calling him the favorite for the Academy Award. And the year seemed to bear that out, as he won plaudits from the New York Critics, the National Board of Review, and the Hollywood Foreign Press. And yes, eventually, finally, the Oscar itself:
That wasn't even the end of his run, by the way. Six months later, The Lost Weekend was one of 44 films selected to compete at the 1st Cannes Film Festival. Not only was the film one of 11 to share the Grand Prix (this was before Palme d'Or), but Milland was named Best Actor - and didn't have to share at all.
But back to matters Oscar. These were the nominees for Best Actor:
Bing Crosby, The Bells of St. Mary's
Gene Kelly, Anchors Aweigh
Ray Milland, The Lost Weekend
Gregory Peck, The Keys of the Kingdom
Cornel Wilde, A Song to Remember
Bing Crosby was back in his Oscar-winning role of Father O'Malley in The Bells of St. Mary's, which, like its predecessor, was the highest-grossing film of the year. Gene Kelly made a surprise splash, and his only nomination, for Anchors Aweigh, a musical comedy about horny Navy men getting domesticated. There's Milland. Gregory Peck was only 28 when he played the Catholic missionary priest in The Keys of the Kingdom, which takes place over 50 years. And, finally, because Oscar law states someone has to be nominated for a biopic, there's Cornel Wilde as Chopin in A Song to Remember.
My rankings, from fifth to first:

