A fun cooperative puzzle game, definitely a nice way to spend an afternoon with family.
A single player puzzle game that gets difficult relatively quickly. It's nice to be able to immediately hop in and spend a few minutes trying to solve a level.
Mario Kart 8 Deluxe
The first game that I bought for my Switch, and definitely worth it. Whether it's with friends or family, single player or online, I don't think you can beat Mario Kart in terms of fun arcade-y racing.
Mario Tennis Aces
I'm not very good at Mario Tennis Aces, but that doesn't mean it's not fun! Online tournaments are a nice feature, but matchmaking can sometimes take a while to find an opponent.
Nintendo Entertainment System - Nintendo Switch Online
A collection of classic NES games, though I normally just end up playing Super Mario Bros.
A 4v4 third-person shooter where all the weapons shoot ink - what's not to love? By far the game that I've played the most of on my Switch, and I don't see that changing in the forseable future.
Super Mario Maker 2
A really fun game! The story mode has a large number of really good levels, and the online mode has (as good as) infinite levels. I'm not quite creative enough to make anything decent in the course creator, but the rest of the game is more than worth it alone.
Super Mario Odyssey
The main 3D Mario adventure on the Switch. I love the game, but I can't control it anymore after getting used to Splatoon 2's control scheme 😛
Tetris, but you're against 98 other people. Normally this doesn't change the game that much and so it's enjoyable as a normal Tetris game too (there's also a DLC item that adds a classic marathon mode if that's what you're looking for).