On the multiplayer side Smash Ultimate is a must have if you have an interest in fighting games. Splatoon 3 is also pretty good if you want an online multiplayer game (just keep in mind that you'll need a Switch Online subscription !). And for singleplayer Super Mario Odessey is one of the best game on the console. If you don't mind longer and story-oriented games, 13 Sentinels Aegis Rim is one of the best science-fiction story I ever played/read/watched. Persona is also a solid recommendation for the system. If you can start with Persona 5 Royal, but starting with Persona 4 Golden is fine as well (there is no need to play the games in order, and both are newbie-friendly). And for a general tip I give to new Switch owners : use Deku Deals instead of the eShop when browsing for games. Here you can compare prices on different sellers and get notified by email when a game on your wishlist goes on sale.