IV: Rydia, for reasons I can't quite remember because it's been a long time since my experience with IV. I like her design at least.

VI: Kefka. The game has no shortage of amazing characters, but the world-destroying psycho clown tops them all. Without a doubt my favorite villain in the FF franchise, and I'm a sucker for great villains, so he narrowly beats out even Terra for this position.

VII: Tifa. Possibly the most badass woman ever put in an FF game and one of the most badass women in all of gaming. She doesn't need a weapon or an array of magical attacks -- she just takes on soldiers, dragons, and eldritch abominations with her fists and a few wrestling moves. Also bewbs

IX: Vivi. Garnet comes close, but Vivi (so far) has the best character arc and personality. Also, he's so presh.

X: Auron. A complex and engaging character like a lot of the X cast in concept, but not an obnoxious tool like a lot of the X cast in execution.

XII: Can't pick between Fran, Ashe, and Vayne. Simply because Fran and Ashe are both awesome characters and badass in their own special ways, while Vayne is a great antagonist who was doing terrible things for logical and sympathetic reasons.

XIII: Fang, undisputedly. Pretty much the only character in that game who stayed great for me the whole way through and never dipped into obnoxiousness, and she had the cutest chemistry with Vanille.

XIII-2: Hope in his manly adult man form, because he was just so damn lovable without being obnoxious.

Lightning Returns: Bhunivelze, because by that point I could really empathize with his desire to destroy the XIII universe and Lightning with it.

XV: Ardyn. A cool, competent schemer who plays the party like fiddles to accomplish his grandiose plans without coming close to the tropes I hated about Caius Ballad doing the same, and now one of my favorite FF villains overall.