i think it was X.

X had the best difficulty by far IMHO. battles like Yunalesca, Seymour @ Gagazet, Braska's Final Aeon were very challenging and the bosses were just plain difficult, not cheap. the game also did a great job at making the battles feel very epic.

I like how you could switch in and out of battle, i liked how aeons were playable, i liked the turn based system, i liked how you could see the consequences of using a powerful move (an overdrive would take longer to recover from than say a regular attack). Overall, i just thought this was the most strategic FF game to date.

VII was a great game, but it was stupidly easy

VIII ^^^ same as VII^^^

IX was my favorite game, but again too easy.

XII was too easy plus i didnt like the gambit system

XIII is just annoying