Rain Man is one of the few films to go down as the box office champ of the year and win Best Picture. Other films that can claim that title include:
Lord of the Rings: Return of the King
The Sound of Music
My Fair Lady
Lawrence of Arabia
The Bridge on the River Kwai
The Greatest Show on Earth
Going My Way
Gone with the Wind
Mutiny on the Bounty
Now, it’s much harder to earn both number one at the box office and win Best Picture. The chasm between what sells and what Oscar voters like has never been wider in all of Oscar history.
Other interesting things to note, both 2001: A Space Odyssey and The Graduate were number one for the whole year. Imagine such a population of moviegoers. This, according to Wikipedia.