Okay so let's just say I've played almost every main series game. The only one I didn't was XI because at the time I thought the idea of a Final Fantasy MMO was dumb. (Meanwhile now I constantly play FFXIV).
Let's do this in order of favorite to least favorite.
FFVI:
This game is pure gold, it honestly is. All the characters have SOME point (except maybe Umaro and Gogo lol) and their stories intertwine throughout the entire game, and even deeper if you get the side missions done. Shadow dreams anyone? You should have expectations of this game considering it was the last Final Fantasy title on the SNES, and it meets all of those. The sprites are beautiful and the music is stunning. They wrote a god damn 3 movement opera for this game and a 4 movement final boss fight, both around 15 minutes in length. As for the villain, he literally become an incarnation of God, nearly achieving all of his goals. No other boss has been as sick and sadistic as Kefka.
FFIX:
As was said earlier this game is art. Another "last on the system"game for the series, Final Fantasy IX has a really compelling story and a clever and despicable villain. Some of the character models could have been done better imo but it was still top notch. (Trivia Time: This was actually my first Final Fantasy game.
FFX:
I absolutely love the cutscenes in this game. They're the best out of any Final Fantasy yet. Most specifically the Tidus/Yuna water one (if you've played it you know what I mean). The villain made you literally hate him. He was so cocky and so snide, and the worst part is they let you play as him and make him seem like an alright guy until he shows his true colors. The music is by far among my favorites.
FFXIV
My most recent MMO addiction and it's a ton of fun. It looks beautiful and the music is well done. It's difficult but not too difficult if you can learn the raids.
NOTE: At this point my favorites get less specific.
FFIV:
I enjoyed this one a lot. The character were really interesting.
FFXII:
I saw some complaint about this title earlier, but personally I liked it a lot. I feel like the license system is something you either love or hate and I loved it.
FFVII:
Honestly a large reason of my strong dislike of this game is the fandom. It does drag down the title for me unfortunately. The game itself WAS revolutionary for it's time but it really doesn't hold up. The story was alright. (I should note however that I absolutely adored Crisis Core.)
FFI, FFII, FFIII, FFV:
I kinda group these all together because I wasn't massively impressed by any of them to be quite honest. Honorable mention for FFII for having perma-deaths (until the GBA versions) and giving me legit feels. Play all of these on their GBA or DS releases if you haven't, they're better than original.
FFXIII:
Let me tell you a thing. Having your 2 main characters be absolutely boring to the point where I don't care enough about them to see their development throughout the game is a bad idea. Honestly I wish I could say I like this game, but I can't stand Lightning and Hope. The game looks beautiful, and it has great music. I love the side characters, but they just don't carry Lightning and Hope for me. (Also let me tell you how much I absolutely hate Lightning Returns for taking anything redeeming about Hope and Lightning and removing it completely in order to "make them more efficient". FUCK that noise.)
FFVIII:
Last and certainly least, FFVII. It wasn't well put together imo. You can get the most powerful magic before the first boss and the most powerful weapon before the end of the first disc if you grind enough, and while it does take a lot of grinding to do, I just don't appreciate that. I will say the fact that enemies properly scaled with you was really cool though. It also doesn't look very good and the music is meh at best, with some exceptions.