Fuuuck.... just one? I've got at least 4 that I feel people should play. I guess, if I had to order them and choose my favourite, it'd have to be The Binding of Isaac: Rebirth. I've sunk 420+ hours into that game and its expansions, but due to the subject matter and disturbing-to-some storyline (Isaac's mother is trying to sacrifice him in order to appease a voice in her head which she interprets as 'God'
, it's not for everyone.
The other three would be, in order, Undertale, FTL: Faster Than Light and Mr. President. FTL because it's the perfect mix of random bulls***, difficult choices, rewarding and engaging gameplay and an ever-changing set of galaxies. Even if you pick the same ship, you won't have the same experience twice.
Undertale is... well, it's f***ing Undertale. It was considered Game of the Year for 2015, achieved a
Perfect score on Steam... and it was made with Game Maker, a fairly basic game-making program. Whether you want to or not, if you play this game, you will get a serious case of the feels. It's one of the few games that actually make you feel bad for killing the generic bad guys and encourages you to take the peaceful route. As one of the taglines so accurately put it "It's the only RPG game where NO ONE has to die!" And that includes bosses.
Oh, and this guy here is a little f***er who deserves to have every petal slowly plucked from his stupid little smiling face!
And Mr. President because the whole premise is about protecting 'Ronald Rump' from getting assassinated as he tries to become president... Or you could just watch him get shot.
You play as a bulletproof bodyguard with a serious case of ragdoll physics who can't even climb down stairs properly. But you'll have to run, jump, dive, tackle, stick your a** in Rump's face, etc... whatever needs to be done to save the future dingus-in-chief. And then you can dance about it afterwards.
I know you only said 1, but I had 4. Sorry about that.