Now that I've played enough of VIII to realize the hate for it is mostly due to Spoony, I would argue that IV is probably the worst game in the main series.

I like Edward as a character and find it annoying that the game treats him as pathetic for weeping over the death of his lover. Especially egregious is Rydia telling him that she's stopped crying... then crying later on when Cid and Yang (two people she barely knows) "die."

I hate Rydia. I hate Gau. I hate Strago. I hate Lightning. I hate Yuna. But most of all, I hate Ashe. SO much.

I will never understand Square's fascination with the "perky young girl" stereotype.

James Taylor Arnold is a damn good voice actor. It's the voice direction in X that's atrocious. Ditto Yuna, Seymour... pretty much everyone except Rikku, as she's still terrible in X-2.

What else...

I love VII, but it takes itself way too seriously.

I hate side-quests that contribute nothing besides more battles. (Marks in XII, Monster Arena in X, missions in XIII)

I miss pre-rendered backgrounds. =(