SNES
Best: FFIV (I played the DS version). For awhile I actually thought this might be my favorite FF, but I think there's a liiiiittle something missing in it for me. A warmth. But it's really good, and I love Rydia, and omg the song that plays in the Sylph cave.
Worst: FFV. While I'm not a huge fan of FFVI, at least the character design of Terra is smurfing GORGEOUS. I badly want a glam of her outfit in FFXIV. That said, I've given up on FFVI about an hour in so many times. But FFV? I couldn't make it past the first 15 minutes.

PS1
Best: FFIX. This game is almost perfect for me. I do find the first four hours or so (until you beat the Festival of the Hunt and leave Lindblum) slow and kind of dull, but after that...the locations, the music, the story. Beautiful. And just a lot of smurfing fun.
Worst: FFVIII everything about this game is terrible. Well, except Moombas. Moombas are great and I wish they'd show up in other games. The rest of this game can die in a fire, tbh.

PS2
Best: FFX. After FFVII when I was a child, this is the game that re-introduced me to the series. Even though FFIX is my favorite, I consider this to be the objectively best game in the entire series.
Worst: FFXII. I think this game had some real potential, and the music is great. The story is just a smurfing mess, and I really wish that the original director had completed it.

Current
Honestly the only FF game I've played past PS2 is FFXIV, so it's going to be my clear choice. I love this game.
The game I'd be least likely to play and most likely to hate is FFXV.