I respectively disagree partydevil. I believe square enix has done a find job of evolving the combat system. The story is a different matter, but let me break down briefly, starting with FFX. And I'll get more in depth if you me to.
FFX
Story: Awesome.
Characters: Great. I feel like every main character was well developed.
Battle-System: Great. SE perfected turn based combat with this one, imo.
FFX-2
Story: Ok. It was fun and yes, it was very girly. But it at the same time, it felt insignificant after all that happened in FFX.
Characters: It only had a few compared to FFX, but none of them really brought much to the table. Payne had her little story, but I didn't really care.
Combat: Fun fun. Timing and coordination was very important, and the battledress system added a cool new element.
FFXII
Story: The story was strong, but the pacing was awful. I can remember going through entire zones without a hint of dialouge.
Characters: Some of my favorite characters are in 12, but that doesn't entirely make up for the fact that most of them were very underdeveloped
Combat: I never tinkered that much with Gambits to be honest. I controlled everything manually, so it really felt like a mix of X and X-2. Which was great!
FFXIII
Story: Once you move past the barrage of new terms, 13 has a good "us against" the world story. There are some odd moments of logic, but it was good all in all.
Characters: A good cast. Everyone had the spotlight atleast once, and each character developed well. They could still be annoying at times...
Combat: Fast and fun. It really only opens up towards the end of the game though, and on harder opponents.
FFXIII-2
Story: Too outlandish, imo. SE had a good foundation for a story after 13, rebuilding civilization on a hostile planet. Instead, we get some wierd time travel junk.
Characters: Didn't care to be honest. Aside from Caius being a good villain, there wasn't much for me to like.
Combat: A subtley improved 13 system. The new monster collection system added a nice twist, but I would have rather have had a third party member.