Gonna go with Bayonetta 2. That game really demonstrated to me what the genre could do and seemed more 'next-gen' than pretty much anything on the PS4 and XB1.
MK8 was also mind-blowing, and if it released the way it currently is on Switch it would win easily, but the original 2014 release had that terrible battlemode and needed to start getting DLC packs to reach its full potential. Smash 4 was great fun at the time, but it lost its sheen very quickly compared to other Smash games because of the lack of good single-player content and because it was missing that element that brings it all together that the other games in the franchise have.
Shoutouts to Wolfenstein the New Order for just being such a fun shooter and Middle Earth Shadow of Mordor for having an actual innovative mechanic in the nemisis system that showed the 8th gen didn't have to be as boring as it was in 2014.