Raiders
Last Crusade (Close,I think Raiders still beats by a hair,but I think LC has the better Maguffin)
Temple (always loved the dark tone.Gets a little slow mid movie -at least by Indy standards)
Kingdom (I don't think it's a bad movie,though definitely a cut under the other three.Spielberg said he wanted the film to be a "blood relative" of the other three,and I think he was successful.)