Which beginner are you buying for?
Someone who's never held a controller. Start with Nintendo Switch Sports. You swing the controller like a racket or a bowling ball and the game does the rest — there are essentially no buttons to learn, which removes the single biggest barrier for a first-timer. It’s also the reason it works so well with older relatives.
Someone who finds games stressful. Animal Crossing or Stardew Valley. Neither can be lost, neither has a timer, and both reward pottering about at whatever pace suits. These are the games that most often change someone’s mind about the whole medium.
A child, or an adult who wants a proper adventure without the frustration. New Super Mario Bros. U Deluxe is the specific recommendation here, because it includes characters designed so that a struggling player effectively can’t die. It’s a real Mario game, not a watered-down one — the safety net is optional and invisible if you don’t need it. Kirby does something similar with a very forgiving difficulty curve.
The two-player shortcut
If you’re going to be in the room, pick something with drop-in co-op. A confident player alongside a nervous one is the most effective beginner tutorial ever invented, and it’s far less patronizing than an easy mode. Mario Kart, the Mario platformers and Kirby all support this.
What to avoid giving a beginner
Three categories, all well-meant and all counterproductive. Competitive online shooters — they’ll be matched against people with thousands of hours of practice and have a miserable time. Huge open-world games — the acclaimed ones are wonderful, but a hundred-hour game with a map full of icons is overwhelming as a first experience. Anything described as “challenging” or “punishing” in its own marketing — that’s a genre with a devoted audience, and a beginner is not in it.