
I think a big reason Raiders and Last Crusade are the best in the series is they have the best "MacGuffins".
There are lots of reasons why Raiders of the Lost Ark and Indiana Jones and the Last Crusade are considered by many as the best Indy movies but I think a big one is they both have the best artifacts at the center of the plot. Each is perfectly set up in an early exposition scene and then paid off in the climax. Compared to the other they're easy for audiences to understand. The ark is a chest that when opened unleashes destructive power while the Grail is a cup that gives whoever drinks from it eternal life. You get why the characters would go to great lengths to find them.