Feeling like a total casual I voted Pokémon Go.
That thing was like the second coming of Christ...Pokémon. My old aunt played and walked right down in a swamp and dropped her phone chasing a Snorlax.
Events gathered massive crowds just playing the game. The first time since LAN-parties in the late 90s early 00s that I was around that many other people playing a game, and in the outside of all places, the mortal enemy of gamers.
The game might not be that special, but the experience it brought was unique.