I will say rocket league. That game came out of nowhere and boom, over 5m download. It also has a good metascore. Splatoon had a similar promotion budget to mario kart and also had a multiplayer demo which seal the deal for a lot of gamer. 

Edit:  RL also had a prequel that almost no one knew about. 

Interactive games dont count.

Captain Toad? That released this year in Europe if that counts. Considering its a small, low budget pick up and play game, the sales have been very good.

Lets not start with that k?

Wait wait wait.... there is a Mario tennis game coming out this year? Holy cow! Love the series ☺

To the google-mobile!

problem with the word essential treatment was gamers are pessimistc about the game, most likely those non Wii U owners.  Splatoon is one of the sleeper hit contender i would say.  it got an impression in a negative way and yet it became a hit to gamers who like a different type of shooter.  I wouldn't discredit Rocket League that it was a sleeper hit but it is free right?  

you would say a game (Rocket League) is a sleeper hit due to 5m million downloads, but it is essentially free for PS+ members.  I would say it is fun and a contender as well. But Splatoon is also a sleeper hit this year.  nobody wanted that game to be successful and even several wii u owners doesn't seem to buy this game.  multiplayer demo is a common thing for shooters, so what do expect, they needed info about the stats on the game.  how the servers would handle it, optimize this and that. 

Rocket League, Splatoon would really be contenders for the sleeper hit of the year.  Until Dawn, I don't know, maybe for some and i believe the sales are good in US and EU to merit it as a sleeper success.

It was the big reveal of Nintendo's E3 in 2015!

well, i couldnt get trought the whole think u know... Dont blame me, the amibos/skylander talk was boring as hell

Tearaway unfolded