*All opinions on this post are based on the original versions of each game, except XI, which now has a Trust system.

NES Era
Best: I. Sure, it was buggy, lacked auto targeting, and not very complex, but the game has a charm to it and it's not hard to figure out where to go and what to do. It also started the series, and who knows if we'd have any of the others if that didn't happen. It beats III due to III's incredible difficulty in the last quarter of the game.
Worst: II. A lack of auto targeting, doors to empty rooms, high encounter rates, and bad row mechanics hurt this game badly. When there are things like putting magic users in the back, but limiting magic points and having all attacks fail makes it a mess. If you do equip a bow, you get a huge stat penalty.

SNES Era
Best: IV. The soundtrack is excellent with plenty of great tunes. However, this game does the best job of making the characters' battle roles a part of the story. It crosses over storyline and gameplay flawlessly.
Worst: V. This is a fun game to play and there's so many ways to play it. It's a great game, but IV and VI are excellent. It's like the Bayley to Charlotte and Sasha.

PSX Era:
Best: IX. It's a fun, goofy adventure with characters that are a mix of humans and non-humans. It's just the right mix of light and dark moments.
Worst: VII. This game is heavy throughout, but gets really gloomy at points. The gameplay is mostly fun, but there's a lot of temporary genre changes (like a motorcycle fight game). However, what annoyed me most is that the direction of the button you press often doesn't match which way your character on screen goes.

PS2 Era
Best: XII. This game is fun. I'm a huge fan of the gambit system. Instead of constantly entering commands over and over, I can just set of list of things I want them to do and they carry it out. The system has a ton of flexibility and I really like the idea of MP replenishing as characters walk around, so running out of MP is more of a time penalty than a hard, run back to an inn, issue. XI has similar play, especially now with Trusts, but it takes forever to do anything in XI, so XII wins.
Worst: X. This game annoys to snot out of me. I can deal with Yuna and Rikku, but everyone else is really annoying. Auron is especially bad, as he acts like a know-it-all, pushy jerk quite often. The cutscenes completely ruin the flow of the game and they're unskippable. Voice acting just became a thing and it shows in the quality of it here.

I'll skip the PS3 era, as I haven't played XIV yet and haven't finished XV yet. I do plan to do both someday, though.