I would have to give my 2015 Favorite to Splatoon.
Most of the time when I play multiplayer games, it ends up in a mess of frustration. The only game that I can honestly say that I've really had fun was TF2. That is, until I got Splatoon. Like TF2, it's cartoony, simple to get into yet hard to master, and can be as casual or competitive as you like. Of course, it can be infuriating at times, but there are ample moments where I'm actually having genuine fun. Basically, it reminded me that multiplayer games can be enjoyable. I only stopped playing it because I know of no one else who owns a Wii U. Oh, and the lag can be a pain at times. Maybe I'll get back into it when my winter break starts.
My runner up would be Undertale. While it is a good game, it's also overrated.