As a rule of thumb, any game that receives major critical acclaim upon its release is overrated. One cannot make a substantial analysis and evaluation of game's qualities during a single playthrough, let alone 48h or so within release (I think some journalists receive a copy early, but that doesn't change much). Not to mention those games which receive a shitload of awards even before they are officially released.
However, just because a game is overrated doesn't mean its bad. Quite on contrary. Many games which have been rightfully mentioned in this thread as being overrated are fantastic games on their own, but don't deserve all the praise they get.
@Kohno mentioned The Witcher 3, which fits this quite well. Even though I enjoy the game and love the series, it is admittedly a very flawed title which may be one of the best RPGs on the market currently, and one of the best aRPGs ever, but is far, far from perfect.
Oh, and did I mentioned that every open-world game is overrated? The whole concept where you need a vast world to explore in order to properly immerse yourself in the game. I call that bullshit. Some of the most immersion-breaking games have been open-world, such as mentioned TW3.
Overrated concept which, unfortunately, will never die out.
Unrelated to what I previously stated, I have to say that I consider every multiplayer-only game that serves as nothing more but a platfrom for e-sports overrated. This especially goes for MOBA. If the game's sole selling point is a single arena where players are supposed to clash, regardless of a number of available characters...well, that's a shitty game in my eyes with artificial depth and little substance.
The overrated aspect comes mostly from its popularity, where unfortunately such games influence the development of many other titles which are not even in the same genre.
Case in point - League of Legends. Its art style has become cancerous. Just check out Steam and how many games are inspired by the looks of LoL. Disgusting.