Overrated simply means that the person making the judgement thinks the game was not as good as the general consensus about it said it was.
IE - FF7. Many people regard that as the ultimate RPG, the best RPG ever made. Personally, I thought it was just ok, not great, not garbage, just ok. That means that as far as I am concerned, the game was overrated.
As for what has been the most overrated this console generation .. I could not say. I don't give a rat's arse what the ratings say, and I never look them up.