I've only played a few - X, X-2, XII and the XIII trilogy - but my least favourite so far was probably XII.

I really liked it, but it felt like it went on a BIT too long.

Also, quite a lot of grinding was necessary to defeat the final boss. I don't mind having to grind to defeat optional bosses, but you ought to be able to defeat the main storyline boss pretty much by just playing the main storyline.

The combat system did start to bore me a bit in the last quarter, as well.

All that being said, I still think it's a really good game, which I very much enjoyed playing.