Raiders of the Lost Ark would most certainly be first, as it is on another level when compared to any of the others. I just watched it again on Saturday evening, and it was still fun and fresh, and I never get tired of it. Definitely the best of the bunch by quite a margin.
Indiana Jones and the Last Crusade - Excellent flick, and the final set piece is one of the most memorable aspects of the entire series, what with the creative traps, the old guardian, and the music!!! The Old Knight's theme is so memorable, and one of my favorite pieces of music in film, ever. The nazi characters aren't as memorable though, and the Elsa is no Marion Crane. Still, a top notch adventure that recaptures the bravado and fun of the original.
Indiana Jones and the Temple of Doom - Not as much fun as its bookends, and Willie is damn annoying. That said, I still like this one a lot. Shorty doesn't bother me quite as much as he seems to bother some folks, and the opening scene is right up there with the best the series has to offer, but when all is said and done, this film's flaws bring it down a couple notches, holding it back from classic status in my eyes.
Indiana Jones and the Kingdom of the Crystal Skull - Booooooooooooooooo.