I can't really say about the NES because I only played FFI and it's been years ago.

SNES
Best: Final Fantasy VI - It's still my fave. The story and and the characters are simply amazing. And also the music. That game doesn't get old for me.
Worst: Final Fantasy V - I love that game, but I had to choose a worst one for the SNES so here it is. I'm not particularly fond of the job system, but the story and the cast are great.

Playstation
Best: Final Fantasy VII - It was hard to choose between FFVII and FFIX because I really love the story and the characters for both of them, and also the music, but I prefer FFVII's gameplay with the materias.
Worst: Final Fantasy VIII - The story is great, but I don't particularly love the characters in that game, and I'm not a fan of the junction system either.

Playstation 2
Best: Final Fantasy X - It's my second favorite FF game. The story is absolutely amazing, and the characters are great too. There are also many beautiful songs like A Fleeting Dream or To Zanarkand.
Worst: Final Fantasy XI - I'm just not a fan of MMO.

Playstation 3/4
Best: Final Fantasy XV - The story is very addictive, and the characters are very interesting too. Noctis is one of my favorite FF protagonist. And the music is beautiful.
Worst: Final Fantasy XIV - Like I said for FFXI, I don't like the MMO.