I'd start by checking out Game Pass, there's plenty there already.
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.
You can get a month's free game pass, that has a bunch of X enhanced games including halo 5.
Tomb Raider and Sea of Thieves will be on there to.
Forza Horizon 3 is amazing in 4K
There are a few game with full game trials too, Tomb Raider and Ghost Recon Wildlands both do I think.
Forza 7 has a demo you can download
Fortnite and Warfame are both free downloads . Fortnite has just had a big update and runs 4K/60*
Titanfall 2 looks amazing at 4K also.
Many thanks! I'll probably grab Titanfall 2 and Fortnite for now and save the rest. Gamepass as well since I really want to play Rise of the Tomb Raider and Sea of Thieves.