It wasn't unique to 2015, but 2015 deserves some credit for reminding me just how fun it is to shoot stuff while hanging out with your friends. Helldivers is the game that woke me up to the fact that I actually had a friends list on the PS4. Before its release I was isolating myself with console exclusives and never looking up for camaraderie.
Arrowhead brought it hard with Helldivers. I assumed that fantasy was more their thing but the space marine action won me over. Of course, tying into the patriotism of Starship Troopers is going to win nostalgia points with me, but contributing to a universal cause is a great carrot. The enemy factions are smartly designed and being able to choose what enemies I wanted to shoot and customize my load out for was just the right of customization that this space war needed. The challenge to take on harder missions solo was just as addicting as casually grouping up with friends to cause true alien devastation. Ironically though, it wasn't necessarily the co-op that kept me teaming up in Helldivers.
I kept coming back to Helldivers for the glorious and aggravating accidents. I admit I like to distract my friends and see them crushed by transport ships almost as much as I enjoy panicking during battle and shooting them in the back of the head. In my defense, if I hadn't shot them I would have just ran off to save my own skin and they would have ended up being slaughtered by aliens. It's better to be killed by a friend than an alien, Helldivers agrees with me.