Probably Undertale.
I came into the game having no idea what it was about, thinking it would be some short, weird 8-bit game about memes or something.
Boy was I wrong, and it was one heck of a ride.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."