Do you have any Xbox One games already? Because it depends on that!
If you are into Racing I would say Forza Horizon 3 or Forza Motorsport 7.
If you are into adventure gaming I would say Assassins Creed Origins or Rise of the Tomb Raider
If you are into RPG I would say The Witcher 3
If you are into shooters it becomes difficult what to recommend because I did not play through anyone of the available ones yet.
Some of the games above should be also cheaper by now as they were released some time ago.
Shooters is too diverse a genre.
Enhanced on the X is (games I've played) : Titanfall 2, Overwatch, COD WW2, Destiny 2, Battlefront 2, PUBG, Fortnite, Wolfenstein 2, Halo 5
I'd say that Overwatch is worth a look for someone into it. Battlefront 2 looks goddamn amazing on the X and is really fun.
PUBG is an essential. Fortnite is also fun and as its free worth getting to see whether you prefer it over PUBG.