It's probably still Tetris 99. I've heard some people complain about the game's controls or the way it works, but from the very beginning I've found it a pleasant experience. I've only had the Switch since January this year, and Tetris 99 is my most played title at "135 hours or more" of gameplay. It's followed by Breath of the Wild, with 100 hours, and Mario Kart 8 Deluxe and Splatoon 2.
Lately I've played a lot of Splatoon 2 as Turf War is pretty short and fun, so it might surpass even Breath of the Wild if I don't continue my second playthrough of it soon.