Oddly enough, FFI. It was great for its time when other games were doing "you saved the princess and the kingdom", this game gave you a "You saved the world but now no one knows you did it meaning you will die in obscurity." It just seemed the most realistic to me. It's a bittersweet ending.

Beyond that, I actually don't care for the majority of the series endings. I actually don't care for happy endings cause they are predictable. If you know its going to end happily, you can just look at each character and make a pretty easy guess as to what happened to them. The few that don't fall into the happy ending scenario usually had some issue (mostly I disliked the cast) that made enjoying the ending difficult.