As far as worst FFs go, its probably no surprise I'm going to point to FFXIII. Linear to a fault, unlikeable cast of characters, fairly simple story made unnecessarily complicated by confusing terminology, battle system that plays itself.
I totally agree with you on some points, yet totally disagree on others.
It was linear, way too much indeed, but the cast of characters were pretty good from my view. Lightning and Fang we're badass, (and cute), Serah was clueless but I think it was the point. Snow, unlike in FFXIII-2 where he looks quite weird, was okay. Sazch was a funny fellow. Not much to say about Hope and Vanille beside that they were fine.
The story was nothing extraordinary, I will agree on that, but far from being the worst of the series. It was good enough to keep me interested.
As for the battle system that plays itself, thats the other point where I totally agree with you. It surprised me, because that actually one thing that most of the people though it was good on this game... I would have call this system "Auto Battle", because thats almost what you do all game and its really boring.
Things i'd like to add : Awesome environments/cut scene and lack of towns/villages/npcs.