XIII, because it was bad on nearly every level. It didn't even have a usable map for god's sake.

A more constructive answer: VIII. Seriously, I love nearly every mechanic and setting in this game except for the way it all got put together. The Junction system had so much potential but Square kind of shot themselves in the foot by making high-level spells so easy to acquire. Every time I want to go play it, I hold off because it'll just be so depressingly easy. I could always, like, not go for high level spells right off, but then I literally wouldn't be playing the game properly. FFVIII is like the definition of poor balance.

Setting-wise I loved it, but I just wish the towns felt like a more cohesive world with each other. I love places like Dollet, Deling City, and Winhill, but they all end up feeling very isolated from each other in the way the game kind of carts you along to one place after another and never really brings you back to any of them. Also, because the game is structured a little more hub-like (rather than straight-ahead adventuring a la VII) I never really got a good feel for the world map. Then there's the whole thing with the story falling apart after disc 2... and the orphanage... and Ultimecia... and... oh god I'll stop. I still love FFVIII but I don't really know why..