Honestly, with the exception of moving around and combo bonuses, there really is not much difference between FFX-2 and FFXII's battle systems. Hell, technically FFX-2's system is simply a grossly sped up version of the original ATB system from FFIV before FFV came around and streamlined the system for players.

I like all the systems, but if I had to stick with only FFs versions of the systems, then I would say ATB is the best because the Turn Based systems have some clunky mechanics to them, CTB exists in a game that rarely made use of it and squandered its potential, and I feel ATB allows for some more flexible and inspiring game mechanics (like player input controls) that ADB just can't really implement. CSB just doesn't do anything for me, its all speed with no real substance to it in terms of strategy and control.

So ATB is my choice, if I could count RPGs in general I would say Turn Based to be honest because Press Turn system from Shin Megami Tensei III and Digital Devil Saga might be one of the best battle systems I've ever played in an RPG.