Raiders is an easy pick for me. The Last Crusade was good but I didn't care as much for Indy's rival in that movie as I did in Raiders. I thought in Raiders that Dr. Belloq held very similar beliefs and reverence for the treasures they both seek but that the methods and end games were different and conflicting thus providing a damn near perfect mirror Indy.

That said Temple of Doom is a horrible pile of trout. Crystal Skull was a bit of a mess. It probably would have been a fine movie had it been made 15 years before they actually made it. Hollywood has changed and Spielberg had changed too much as a director...I mean...Indiana kill no one in the movie and the bad guys were barely bad as well. I know that is kind of petty in a way but at the same time I feel it reflects how they treated the material and it grew too far apart from its pulp roots.