Sea of Thieves. Since day 1. Best Gamepass title. Easy to leave and return to.
Deaths Door is a minor masterpiece.
Tunic is a major masterpiece if you want something deceptively cerebral (looks like a kid-oriented Zelda-esque adventure. It is not, I assure you.)
Frost Punk is a very fun city builder with unique elements and design.
Loophero is a sort of tower defense but not? Strange game. Strangely addictive.
If you haven't played Jedi Fallen Order, get on it. Sequel is out and it slaps. Fallen Order itself has a better storyline than all the Disney sequels and Clone Wars (yeah, I said it) combined.
Not on Gamepass anymore, but bonus points for any former Gamepass title that was so damn good I ended up buying it when it left Gamepass. And that trophy goes to Crosscode. Maybe the 2nd best single player game I've played this decade.