Most of my favorite RPGs are turn-based or ATB, but that's mostly because I love the characters, plot, and other gameplay elements a lot. Just focusing on the battle system itself though, FFXII was a huge step in a positive direction, and had the most promise of any game in the series. It wasn't perfect, but it allowed you to mold the battle system to your liking, removing some of the most annoying elements of typical RPG random battles. No other FF game has such exciting boss fights, though FFXIII had its moments (despite being an overall regression from FFXII).