Definitely the Last Crusade, but when you look at the original Raiders of the Lost Ark, you begin to see some of the background into it.
Spielberg, being jewish, was basically making a "childhood" movie about sticking it to the nazis and stopping them from getting away from stealing Jewish heritage. The whole series is based off of a 1940's adventure serial. Which is lovely, really.
That's the genius of this series. It was built around nostalgia and made a perfect adventure movie series.