We've had "favorite" and "least favorite" polls numerous times, but let's try to decide which FF from each generation is the best and worst for you! if you want to explain why, that's fine! Main entries only. And XV is counted as the same generation as XIII and XIV (so PS3 and PS4) because 1) rule of three and 2) it was originally conceived as an FNC title, so it counts.

NES

Best: Final Fantasy III - I've grown to like it a lot more now that I used the Geomancer instead of the Magus, but it's still a really close tie with FFII.
Worst: Final Fantasy I - not that it's bad, it's just the first one in the series, so there is really not much plot (which II has, with actual characters and events), and the battle system is not as interesting as III's

SNES (TOUGHTEST CALL EVER BECAUSE THEY'RE ALL EXCELLENT)

Best: Final Fantasy VI - the music, the story, the characters, everything just works so well together.
Worst: Final Fantasy IV - BUT that's only because it set the template that all future games built up on. It also easily has the best constrcuted narrative in the series, with very even pacing and tension in all the right places.

PS1

Best: Final Fantasy IX - a phenomenal return to the roots that has some of my favorite elements in the series, period
Worst: Final Fantasy VII - while a good game, I was surprised with how bad the second disc was as I replayed it recently. It's one half pointless padding, one part insane plothole. It's gloriously broken beyond repair.

PS2

Best: Final Fantasy XII - the world, the characters, and the battle system are just exactly what I want. I loved how toned down the melodrama was and how smart the narrative was.
Worst: Final Fantasy X - I just don't like the whole "LOOK AT ALL OF MY FEELS!!!!" attitude Toriyama's games present to me in general. it's not as bad as what comes next, though. And the battle system isn't half bad, though I do miss Ito.

PS3&PS4

Best: Can't comment yet because I haven't played enough XIV and I can't play XV yet, but from what I've seen so far, XV is the easy contender.
Worst: Final Fantasy XIII - I had to force myself to play it. Easily the most boring gameplay experience I've ever had with hallways all around, including character building, and pointless gating of levels. And the characters are just terrible. The story, while it has some potential, is just all the worst parts of X's sentimentalist crap on steroids. Do not want.