This is hard to say since it depends on who is rating the game, obviously. For example, I think WK overrates XII and Jesswee overrates XIII. There will always be people who love and hate each game.

But I say in general VIII is the most overrated. A lot of people didn't like it, yet somehow its characters and story manage to gain high ranks in various measures such as the EoFF census. Squall was probably the most stereotypical emo hero yet he gets love from many people. Probably because they like to think they are 'cool' like he is. The other characters were kind of a waste since the focus was a lot on Squall and Rinoa, a love story that was supposed to be the theme of the game but wasn't done well at all and is completely outdone by the love stories of IX and X. Then we get to the junction system, probably one of the worst FF battle systems. In fact I would call it the worst besides FFII. Drawing magic was annoying. Junctioning spells was nonsensical. Weapon upgrades seemed like an afterthought. Limit breaks could easily result in broken battles by just keeping your team in low HP. The GFs were cool and leveling GFs were fun so I'll give them that. The time compression thing was overly complicated and didn't make any sense.

Many people hated VIII. I didn't. I liked it. It just wasn't my favorite or 'up there' with the other FFs. But if so many people hated it then why do people talk about it so much? Why does Squall get votes for best character or get into so many user names? Why try to make sense of the plot?