XIII, you can take or leave the story line, but the battle system is so fantastic. In the early stages of the game, it's kinda simple, you don't even need to switch Paradigms really but at a point, the game utterly drops your hand and becomes ruthless and says "i hope you've been practicing, paying attention to this battle system because from here on out, battles will be unforgiving". With crystal points being so expensive, grinding really isn't practical so to progress you have to plan things out and know what works, who does what ect.
No other FF has a battle system so challenging I think. Later on, auto-battle becomes a toss up because you'll start to think of better ways to take down foes for instance, auto-battle will prefer to use veil and and shell before bravery and faith when you really need the latter, but this isn't until at LEAST hour 15.
I really like XIII but with combat being 90% of the game, the fact that the system doesn't even open up and challenge you until halfway thru is a bit much. You get a Sentinel tutorial within 2 hours of gameplay bout you never NEED that role again until 15 hours later on Gran Pulse, but once you hit that threshold it's awesome. I love it.