Here's something I wrote in my LiveJournal:

*WARNING: Possible spoilers for FFI, FFIV and FFXIII!*



Quote Originally Posted by Sephiroth1999AD
Some people say FFXIII is the hardest FF, especially by the time you reach Pulse. While some of the battles were quite hard, I didn't really think FFXIII was the hardest game in the series. Your party is fully restored after every battle, and even if you lose a battle, you can always retry the battle without having to load the game. I thought FFI and FFIV were a lot harder with their long, grueling dungeons, where any battle could lead to your party dying and forcing you to start the whole dungeon all over again (*cough* FFI's Ice Cave, FFIV's Sealed Cavern and Lunar Subterrane *cough*).
So while FFXIII probably had the highest number of Game Overs I've had in the series, it doesn't really matter since you can always retry the battles.