If I were to say that Empire Strikes back was my favorite because it has the best surprise I've ever experienced in cinema, that would entirely disregard the way ANH set that surprise up as such a shocker.
If I were to say Return of the Jedi is my favorite because it has one of the best showdowns I've ever seen in a movie that would entirely disregard the way the 2 movies preceding it set things up for such a climactic showdown. And now that we have the prequels it would entirely disregard how much more meaningful Anakin's return from the Dark Side is to me now that I have seen where he originally came from to begin with.
If I said TPM was my favorite film because it has my favorite Jedi in action (Qui-Gon) that would entirely disregard his eventual impact on the rest of the saga with the way he came back to show Yoda and Obi-Wan how to become a Force Ghost.
If I said AOTC was my favorite film because I love seeing Yoda put aside his cane and kick ass, it would totally disregard the way the other movies refrained from showing that to make that as special as it was for me.
If I said ROTS was my favorite movie because it was the most emotional experience I have ever had in a theater, it would totally disregard the accumulative narrative I had witnessed up until that point that set me up for such an emotional reaction to how all the events unfolded.
It's just too difficult for me.