To me, the first crack in the franchise came about from it’s biggest reveal: that Darth Vader was Luke’s father. And by the time the first trilogy was over, another fissure had appeared: he was Leia’s dad, too.
While I like The Phantom Menace, the whole Midichlorian/The-Force-is-hereditary introduced another and wider one.
Star Wars was no longer about adventure among the stars and its limitless possibilities. It hac descended into family melodrama with painted stars and pew-pew as its backdrop.
The original movie will always be my favorite of all of them because it can stand up by itself.
Forgot to add: ...And the Force was no longer something anyone could tap into with enough knowledge, patience, and practice. It was something passed on to a being and they either had it or they didn’t.
A New Hope is my favorite also, for the same reason you said. It stands up by itself.