Zelda, Mario Kart, ARMS and Splatoon 2 for games. Pro Controller, Dock Sock to prevent scratching by the dock, and a protective case are must-have accessories. A stand to use in tabletop mode is also useful.
And, if you can afford it, the battery that's been recommended here on iMore and on a number of other Switch sites, the Anker PowerCore+ 26800 PD with 27W PD Portable Charger, is really useful for on-the-go use. It allows you to charge the Switch while you are playing it--which most portable batteries won't do.