I loved X's battle system. Best part of the game. The turn-based format with a queue bar up the side required so much more strategy and allowed Square to make the game that much harder early on.

By the end of the game you could just completely breeze through no matter who was in your party, but for most of the first half, if you didn't actively switch and use everyone (except Kimari, he was useless) you'd have a real hard time winning. You also were required to get everyone out there, or they never earned experience. None of the learn through osmosis auto-scaling that worked in the previous games.

Enemies had a ridiculous number of immunities and dodges, such that only one or two characters could dispatch them (but usually a one-shot). They also hit that much harder because you knew the exact turn they were going to attack, and therefore had plenty of time to rectify your low health.