Hardest: Final Fantasy V. I still have only defeated Neo Exdeath once, and I defeated by constantly saving my game state while fighting him. In other words, I have never finished FFV without cheating. FFV was the only game where I consistently died on the bosses.

Easiest: Final Fantasy VI. I rarely die in this game. The only time it ever happens is when I'm levelling up in the dinosaur forest and when I fight the Storm Dragon. I'm looking forward to this "expert version" that io90east is making.