Best battle system?
FFXII - I don't think this is a popular choice, but I loved the shit out of it. It made grinding fun, and they successfully got rid of the random encounters, and all of the enemies on the map really made the world feel alive. One knock against it though, would be the "auto battle" commands. Once you figured it out the right combination, the game basically played itself. I tried to use it as little as possible, but some bosses practically required it.
FFX - Might be favoring it because I'm playing it now, but I think it's the best system of the bunch. Being able to switch between all of your characters during a battle, and change weapons and armor on the fly were great innovations. Having access to all of your party members at one time also made it easier to level them all up together. I always hated that about past FF games, where you would spend so much time with one group, only to get thrown into a scenario where you have to use all of your characters, and half of them would be so under-powered that you'd have to grind them up. This system eliminates that shit. It also helps the story a bit too, with all of them being involved in the quest. Always thought it was funny how in past games you'd choose three or four characters for your main party, and the others would just chill at an inn or whatever, while you're out saving the world.
Love the old-school systems too, but they're all basically the same with some minor tweaks.
Worst one was easily FFXIII. I can't believe they made a trilogy of that shit heap.
