Is it based on most time spent on playing?
If yes, strangely enough, Sega Heroes (mobile) is my 2018 GOTY.
Daily playing because having it in a smartphone made it’s easier to access.
Other game by the same developer, Marvel Puzzle Quest, hold in second place.
But if it's based on gameplay I'd pick Super Smash Bros.
If it's based on art style, I'd pick Valkyria Chronicles 4.
And if it's based on soundtrack, I'd pick Shining Resonance Refrain.