FFI - 7/10. The game that started it all, the game's fantasy world is very interesting, and the game has loads of replay value with the different party combinations that you can choose. I've beat the game 7 times, and no other FF comes even close (followed by FFIV, which I've beat 5 times).
FFII - NES/WSC/PSX 1/10, GBA/PSP 4/10. A good storyline, but the battle system is just unbearably tedious. The GBA version makes the battle system much fairer.
FFIII - 2D version 5/10, 3D version 5/10. A pretty good battle system, but a very grindy game. The storyline is nothing special.
FFIV - 2D version 8/10, 3D version 7/10. The storyline and battle system are quite simple, but still elegant. I really enjoyed playing this game about 17 years ago. It's almost the most challenging FF for me. The 3D version has a nice Augment system, but it loses points for a hideous camera where it's really painful to navigate, unless you play 99% of the game like this. I also didn't like the voice acting of the 3D remake, and I thought many of the remastered songs sounded worse than the original ones.
FFV - all versions 8/10. The storyline isn't anything special, but the battle system is just fantastic, and FFV has arguably the best gameplay in the FF series. The GBA/mobile/Steam versions also have a new translation that adds plenty of color and humor to the characters.
FFVI - 9/10. One of the best storylines in an FF game, even though it stops after the first half of the game. An interesting detail is that the best parts (IMHO) of the game happen during the second half of the game, where the story takes a backseat and the game becomes a non-linear adventure.
FFVII - 9/10 as stand-alone, 7/10 with the spinoffs. A great battle system, and an enjoyable storyline too. Unfortunately, the story becomes quite hard to understand with some of the translation issues, and the spinoffs have changed or retconned the plot so many times that I stopped caring about them and decided to ignore them altogether.
FFVIII - 6.5/10. The love story is nice, even if it is somewhat forced. The Sorceress story, not so much. I still don't buy the Orphanage scene. The battle system has interesting ideas, but it's a broken mess in terms of balance.
FFIX - 8.5/10. A fantastic storyline, possibly the best in the FF series. Unfortunately, the battle system is terribly slow and clumsy, and much of the game is too RNG-based for my tastes.
FFT - PSX 8.5/10, WotL with the slowdown fix 9/10. A fantastic storyline, but the weak PSX translation makes it confusing at times. The War of the Lions retranslation fixes this problem, and the PSP port adds some other nice features too. A fantastic battle system, 2nd only to FF5's (arguably), even if the learning curve is quite unforgiving.
FFX - 1/10. The turn-based battle system is good, but otherwise, I can't really say I like this game. The game is overly linear. After you reach Luca, you pretty much know what you'll be doing for the rest of the game, and there's not many twists after that. You'll basically just watch the story unfold, as you walk through loads of linear hallways with overly frequent random encounters and overly long, unskippable cutscenes. Tidus is the most annoying main character in the series, IMHO, and Yuna is completely ruined by the atrocious English voice acting. Most of the mini-games are terrible, and the postgame is nothing but endless grinding (and these terrible mini-games).
FFX-2 - 7/10. The story is even worse than in FFX, but otherwise, fixes pretty much everything that annoyed me about FFX. A great battle system, and nice, non-linear gameplay. Supposedly, the postgame content is really great.
FFXI - I haven't played it, so I won't rate it.
FFXII - PS2 7/10, IZJS 8/10, Zodiac Age Steam 9/10. The game has a pretty good political storyline, but it didn't grow on me until the 3rd playthrough. A huge, rich fantasy world, and non-linear gameplay with lots of freedom for exploration. The Japan-only IZJS release adds a 3x speed boost, which makes grinding and retracing much smoother, and it adds 12 new classes, where you can choose up to 6 of them. The Zodiac Age release makes the game what it was meant to be, IMHO. 60 FPS, plenty of graphics options, a faithfully remastered soundtrack, easily adjustable 2x and 4x speed boosts, and letting you choose 2 classes for each character makes the game extremely fun to play. With the choice of having 2 classes for everyone, the game's replay value may even challenge FFI's immense replay value.
FFXIII - 6/10. Suffers from many of the problems FFX had, but at least there's no more random encounters, and the game opens up when you reach Gran Pulse.
FFXIII-2 - 8/10. A bit similar to FFX-2 in a way that there's not much storyline, but it fixes about everything I disliked about the first part. The gameplay is a lot of fun, and customizing your monsters is neat too.
LRFFXIII - 7/10. The storyline keeps getting even weaker, but the battle system is fun and well polished, and the non-linear gameplay is great too. Loses a point because of the time limit. While it's never really an issue, it's still a psychological hindrance, and I play games to relax, not to feel stressed.
FFXIV - I haven't played it, so I won't rate it.
FFXV - I'll rate it if/when I've beat the game.