Apart from the obvious recommendations (Mario Odyssey, Mario Kart, Smash Bros, Splatoon), the two South Park games available are insanely fun/funny and are always on sale. Also:
-Doom and Wolfenstein were ridiculously good, considering the limitations of the hardware
-Do you like beat em ups? There's Raging Justice, Double Dragon 1 and 2, River City Ransom (Nintendo Switch Online), Capcom's Beat Em Up Bundle
-Civilization VI is insanely fun, once you wrap your head around it.
-Dark Souls is frustrating as hell, but it's a great game and it's so cool being able to take the game with you, anywhere.
-Golf Story.
-If you liked the old SCUMM games of old (Maniac Mansion, Full Throttle, Monkey Island), Thimbleweed Park is a must!
-Hollow Knight! A game that every Switch owner should try!