The worst (since that is, after all, the question the OP asked) is Flareon, absolutely. Sorry, but that's just the way it is. The only thing it really has going for it is good Attack, but it doesn't have any decent moves to take advantage of that, and even with better moves it'd still be too slow to be useful.
Personally though, my least favorite (an entirely different question) is Glaceon. It's okay as Ice types go, but the design just doesn't appeal to me as much as the others.