X, XII, and XV by far.
X is just a classic, it feels like the last "classic" FF to me.
Excellent battle system, some really great party members (including Tidus), interesting world and world building, the linear design has enough exploration to make it interesting, and an effective and sometimes beautiful story.
-
XII is FF in MMO's clothing and it's weird. Very compelling systems, some great characters, but the balance is busted. About a third of the way in you can easily have bought the majority of every licence bored, and the lack of traditional skills and spells really hurts gameplay. Gambit system is great, like really great. If only the systems that went with it were more interesting... Still, it is a good game.
-
XV.... What a mess. It's unfinished, has a poorly told story, makes waste of some important characters, has a beautiful but entirely pointpess open world, boring quest design, summons look amazing but you might never even see most of them...
The relationship between the four friends is one of the best examples of well written characters in the series. Endlessly charming, funny, heartfelt. The visuals are incredible at tiems (on PC even more so), it has some of the best dungeon designs of all time, it has maybe the best ending in the wnitre franchise.
The battle system, imo, is again messy but you can have fun with it. Going to no damage runs and the most efficient combos shows how this system with a little more care could have been something special.
Overall, it is one of the most prominent examples of a flawed gem in gaming.
-
XIV is good, I got really into it at launch (ARR) and got my Relic +1s and devoured all end-game. Tried to get back into it but the obnoxious post-game quests and dull story that you have to grind through to get to the good end-gam has defeated me multiple times. I'm only just at the second Stormblood quest.
If it didn't have so much obnoxious fetch quest story grind it might replace XII. I know you can pay to skip but I don't want to do that on top of a sub.