I like the ATB in FFX-2 a lot, because of the ability to get combos by timing your attacks. It also makes the battles much more fast paced and exciting.

That said, I also enjoyed the Wait system in FFX, because it gave you time to plan your battles strategically, and concentrate on each character's strength. I also loved being able to swap the characters in and out of battle.

So, I wouldn't say either was best, as I like them both for different reasons, but I do think it's good that Square never stick to one battle system and always give us something new. =)