Birdman

Michael Keaton was nominated for the Best Actor Oscar for his role as the title character (Riggan Thomson).

When asking about Oscars, it's always best to make it clear whether the year you specify is the year in which the film was released, or the year of the Oscars ceremony at which it won the award. I accept that given the way this question is worded, the only correct answer is "Birdman". But when answering questions, you can't always assume that the setter has got it right. If the question had asked "Which film won the Best Picture award at the Oscars ceremony in February 2015?", it would save anyone having to clarify.