An honourable mention to the original, but for me it's between 13 and 6. And 13 was at least fun.

The only credit to 6 was its story, which has the least final fantasy feel to it (maybe the reason it's the best in the series lol). The gameplay was barren, stock JRPG trite and is the first entry in the series where you need only "hold X to win." Completely inexcusable after 5's example. In summary, it's a terrible game. Hell, even the story only really qualifies as the best in that objective "you can only really appreciate it after taking a literature course" kind of way. 7's story was a hell of a lot more interesting in spite of the numerous imperfections.

I can understand the hate for 2, but sheesh I can appreciate that battle system. It's the little stats engine that just gosh dang could.