Although they're in no way "simple", I am tempted to vote for either FFVII or FFVIII, purley because they are easier to understand than most people give them credit for - FFVII especially.

Everyone keeps saying that FFVII lost a lot of important plot due to bad translation, and while I admit that the FFVII translation was by no means perfect - It was by far the plot that suffered the least. The things that apparently needed to be explained in the Ultimania guides were things like how the the Sephiroth Clones are "made", which is blatently explained in the game when Cloud revovers so I have no idea how people missed that one. People seam to miss what happened to the people from Nibelheim too, but I understood it on my second or so playthrough - you just got to talk to the right people in the game.

Although I can probably thank the fact, that I originally used cheat devices for both games - and this allowed me to restart the games almost instantly after completing them, and then sail through them with the "complicated" parts still fresh in my mind.