Well, played all but 9 (didn't get far into 8 due to the emulator taking a dump basically 8 hours in), 11, 13, 14. My favorite has to be the original. I can sit and play it all day without a thought (and have played nearly all incarnations of it and FF2J) and enjoy it. For over 20 years. I picked it up when I was 7 and my dad had gotten his butt kicked by Chaos and gave up the game.
I learned why over-leveling is bad due to FF1. You see, Chaos gets more powerful the more you level. So at 50 (max) he has EVERYTHING your Black Wizard does, Cure4, AND elemental attacks. When I got older I made my first "challenge" to take him out at 50 just for bragging rights. No Game Genie, no glitches, no exploits. Knight, Ninja, White Wiz, Black Wiz... and a dead timebending demon-god.
Story-wise, 6 is alright I did enjoy the scope and points of view but felt there were TOO MANY, so I found myself not caring for any of them. That's where I liked 5, 7, 12 and especially 10. I cared about the characters because you watched them grow. Yes, Cloud was an emo douchebag but you learned WHY, Seriously wouldn't YOU be if your best friend was killed protecting you? I know I did when Mom died. Tidus was annoying (mostly his voice, I mean really, different voice and no one would complain) but I found myself enjoying his quirks and optimism. Vaan was similar to Tidus in the beginning due to similar backgrounds (what a shock! Similar backgrounds producing similar results... alert the media) but you really watched him grow because of his friends AND better his friends. You FELT the friendships in X and XII and that made them better.
It's easy to tear the series down, but there is a reason why people keep coming back.