What games you want will depend on what kind of games/themes you like.
We have a switch (me, dc aged 12 and 8), and the most played games have been:
Mariokart - we all love this, and we’re still competitive over rankings etc
Pokemon lets go - again, we’ve all enjoyed this, and ds in particular (he also has Sword, but dd and I haven’t really played that due to lack of time!)
Animal crossing - another major obsession for ds, and I love it too.
Lego games (we have Harry Potter, City, and Jurassic World) - loads of gameplay in these, so many side tasks and puzzles and bits to collect and go back to complete etc.
Mario party - lots of fun here too, not one I’ve played so much but the dc do, and also with other family when they come to visit
Zelda - these are my favourites, as I’m a Zelda fan from back in the 90’s. I have Breath if the Wild (amazing game) and Links Awakening too which makes me smile as was one I completed with my mum back in the day on a Gameboy original!
Some of the cheaper small games on the eshop can be lots of fun too - Astrobears cost me 89p I think, and has been played loads.
Tetris and Puyo Puyo are worth a nostalgic look, and the 51 Worldwide games is a lot of fun too - I got my uncle hooked on Yahtzee when he was last over!
You need to look at what style of game you want - platform, puzzle, racing, sport, and go from there, taking into account any theme that might be preferred.