I suppose it all depends on how serious you take the movies. For me, Raiders of the lost Ark and Empire Strikes Back are two of my favourite movies of all time and while the other movies after them were not necessarily better (TOD,LC and ROTJ) they were still a good continuation of the theme in a way where they knew and understood what they were doing. With the later sequels (Indy 4 and Ep1,2,3) it seems like they lost touch with what made the original movies so great. I also believe the film crews that worked on the original films are also what made the films so great whereas these days Lucas and Spielberg use almost entirely different crews, which might explain why some of their filmmaking hasn't been quite the same for a number of years.