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.
I'd have to say Xenoblade Chronicles 2.
Bit reluctant at first try but ended up loving it shortly afterwards.
I got so immersed into it that I had to let social media and GBAtemp sit by for 3-4 months!
I like itss story, the characters & plot. The game is a decent one but leaves to be desired in terms of graphics and performance. Occasionally textures glitch up, take a little longer than expected to load properly, and the game overall is poorly rendered and runs at a jaggy pixelated resolution. In spite of that I love it though. Played torna the golden country which doesn't have as apparent bugs and loved it too.
Already have put 560 hours into it.