Indiana Jones and the Last Crusade is probable the best Indiana Jones movie out of all of them (out of all of the old ones that is, have not seen the new one). The Last Crusade has lots of action, mean bad guys, and a story that is told in a very well thought out way.
The way the movie starts out with Indiana and a kid and his first adventure that made him who he is as an adult was genius. From the snakes, to the adventure, to the whip, and the famous hat, you learn the background story of Indiana Jones. Also the research and following the clues was something that I think was missing from The Temple of Doom, and makes scenes more thought provoking and exciting. The final tasks Indiana Jones must overcome, required quick thinking to avoid the traps, to get the greatest treasure of all the Holy Grail.
Their is just something about this movie that makes it extremely entertaining, and has that timeless value that many movies will never have. This movie also has great replay value, its one of those movies you can watch every year and still never tire of it. I guess there is something about George Lucas movies and replay value (Star Wars).