Over the past week I've watched all of the Indiana Jones movies, all for the first time. I wasn't terribly impressed after the first two, which I'd say are pretty average, but I thought that the third, Indiana Jones and the Last Crusade, was quite good. The action was well-done, and the story seemed much more complete. Raiders of the Lost Ark skipped around quite a bit, Temple of Doom was a little more focused, but Last Crusade was a much better adventure overall.
I'd recommend Last Crusade to the other guy in all the world who hasn't seen the Indiana Jones movies yet, but I'd tell him to skip the first two.