Personal opinions, not objectively or anything:

NES
Best: FFIII. Adorbs job classes
Worst: FFII. Nothing wrong with it, it's just unremarkable

SNES
Best: V. So much fun, great characters
Worst: VI. Just kinda... meh towards it

PS1
Best: FFIX. BEST. GAME. EVAR.
Worst: FFVII. I hope Aeris gets stabbed OH WAIT

PS2
Best: X-2. I know you said main entries only but it's so hard to decide between X and XII so... X-2 it is!
Worst: Uhh... XI? Was that on PS2, what

PS3
Best: XIV I guess
Worst: XII I guess but I like it still. It's just certainly not the best game