HARD MODE: Don't say Undertale
I think mine is probably DOOM 2016. I don't know that I've ever had more raw, uncut, can't-believe-how-much-I'm-enjoying-myself FUN than my first time going through that game. I legit was hooting and cackling like a madman while scrambling around blasting demons and dodging fireballs and getting sick glory kills. My first playthrough was roughly 20 hours of some of the purest joy I've ever experienced in my life and I'd give ANYTHING to experience that again.
Honorable Mention: Heaven's Vault. GOD, that game is cool.
The Last of Us, no doubt. Part I in particular because it impacted my enjoyment of games and even my writing career in so many ways.