This is actually hard for me, because I would probably say all of them had some unrealized potential to some degree. I mean outside of FFV, I can't think of a single FF game whose battle system doesn't need some form of balancing or re-tooling. In terms of scenario, I think a lot of them could have used some better expanding in some areas. If I'm going to go overall, I would probably say FFVIII and XIII are the biggest offenders. Both games are filled with great concepts in both gameplay and the scenario and both fall short overall.

FFVIII - Junctioning needs to be re-balanced, its a munchkins wet dream of power maxing with minimal effort. Even its attempts to balance it are so pathetically weak, all they do is restrict efficiency to one play-style and make battles far more tedious than they should be. I agree that VIII's world, including its towns feel very distant to the scenario. Like Flying Arrow points out, I blame it on the game revolving around using Garden as a central hub, instead using the old stand-by journey set-up 99% of RPGs use. Its a nice change of pace, I just wish there was a reason to return to some places beyond doing the Queen of Cards quest. Even some of the mini-sidequests are so random because they involve the map that most people have probably never realized they are there without a guide, and again, none of them really net you anything worthwhile. Weapon customization is a cool concept but it only affects one character (Squall) there is no reason to upgrade anyone's weapons besides his, because the upgrades are not as useful as good Junctioning.

For the scenario... I'll steer clear of the more touchy subjects among fans and simply say that I feel the supporting cast needs to be fleshed out. I mean all of Squall's comrades basically have the same past, and despite all of them being adopted, you only hear about Zell's family, what about everyone else? Why not expand some of the other SeeDs in Garden? I mean who the hell is Xu and why should I give a damn? I would also like a better explanation about what GFs are and what Junctioning is. Who were the Centra? I mean their technology is important to the plot, but for a group of people who were wiped out less than a hundred years ago, you sure don't learn anything relevant about them. You know more about the Cetra than you do the Centra. Why are monsters on the moon? Is it a real moon like we know it, or is it like a mini-planet with its own environment? What are the Sorceress like when their not despots? Do they have their own culture and rules for living in the world of man? If Hyne was never going to factor into the plot at all, why even bother mentioning him? For a character as important to the plot as Adel is, you really don't learn anything about her. How did Esthar become so technologically advanced? What about expanding Odine, or Ultimecia? Why is Seifer so heavy in the artwork and PR stuff, but is ultimately a glorified flunky no better than Rubicant or Borghen? I feel like he was intended for greater things, I mean the love triangle really doesn't exist beyond the concept art. What about NORG's race? Are they simply here to add some colorful flavor to the species on the planet (as well as explain NORG and one scenario with Laguna) or do they actually have a more entwined relationship with mankind and the sorceress?

Then there is FFXIII, as a game, I would have liked more differing skills between characters. I miss not having steal, throw, and gil toss. Skills are the spice of life. I also hate how the game is pretty much set-up to let you use any character. Even Sahz, Vanille, and Snow who only can use half the elements in the game, chances are, the enemy is going to be weak to one of those elements, so what was the point in not making them use all the elements? I mean outside of stats and when they learn a skill, there is very little difference in the roles between characters when you look at the skills. I would have liked more skills or special spells that would differentiate all the characters from each other a little more when using roles that multiple characters share. I would also prefer combat to be slowed down and more methodical, if the point of making them fast is because people don't want to fight minor encounters, its because the combat is lousy, not because the system is tedious, people who don't want gameplay to interfere with plot are better off watching youtube videos or sticking to visual novel games.

I mean, what made games exciting in FFIV, VI, VII, and IX is that you are constantly being assaulted with new skills and characters, that changed battles. Its hard to get bored when new aspects are being introduced constantly that really change battles and your strategy, and by the time these games stop offering new abilities or characters, you have a wealth of customization to work with. You don't get this in XIII, your first high point is having the Paradigm system unlocked in Chapter 3, equipment is in Chapter 4 but it won't be relevant until Chapter 9, of which Chapter 9 is the last high point because that's when you finally get a full party and one new character. There are just long spans of time where nothing new is being introduced and battles become boring cause your just using the same paradigm set-ups over and over again.

The equipment system itself is a disaster because the growth of weapons takes too long, requires way more grinding than is necessary, and many of their skills are not really that useful. There is a great class system with the weapons and accessories, but most players without a guide won't know it because the game never explains it, course half of them are utterly worthless after the first few chapter but it was an interesting concept for the equipment system that should have been expanded and would have given the player a greater sense of customization. Speaking of which, the Crysterium is awful, what it really should have done is mix your stats with the roles themselves, so if you want to raise magic, you have to raise Ravager or Medic, for attack, COM, for HP go for Sentinel and then you could have SAB and SYN be the hodge-podge set. This would have actually made which roles you leveled matter more in the long run and actually give you some control over how your characters developed. Less linear dungeons and maybe some breathing room to actually learn more about Cocoon and appreciate it.

As for plot... Well that can wait for another day.