Definitely II for worst ff title. I just hated how much I had to grind to progress. Magic sucked all the time, enemies were annoying, and even though I beat the game, god help me I can't remember any of the plot. I just rememver the 4 guys who died and then got the add on story for the gba version. Maybe it's because I played a first generation game 15 years after it came out but I hated it.
Close runner ups include XIII and XII. XIII is pretty obvious, the only saving grace being the battle system which I kinda enjoyed abusing. Characters were annoying, especially the aussie bitches, and sazh had such a sad story why the hell was he the comic relief? It didn't add up right. Oh yeah, why is pulse the australian outback, and what does squeenix have against australia? XII...everything that was bad about XIII stemmed from this piece of shit. I rank II higher on the hate list only because I didn't enjoy anything about it. XII sucked everywhere, except for the gambit system and fran. She was definitely the greatest pixelated piece of ass on ps2. Scary, but true.
The rest of the ff titles all had good points and some bad points. VII was probably the leaat flawed of them all, and VII the most before being a bad game. Spin-off wise, tactics is king, followed by crystal chronicles, then the others were meh to okay. Worst was definitely without question tactics advance. I hated progressing through it, and those god damn judges could eat my asshole, fucking arresting my characters.