Mine is most definitely Splatoon 2. Trying it at my girlfriend's house was what made me get a Switch in the first place.
I must mention, though, if we're talking about games that are multi-platform as well, I absolutely love playing Team Sonic Racing and Death Road to Canada on my Switch. Both are great games you can totally zone-out with, something you can play while waiting in line or something, focus on and get super into, or chill with a friend with. Super versatile and fun.