Problem with the "best games" is many of them have been outdone by their current generation sequels/remasters/HD editions.
On the below list, for example, the only games that haven't got a Remaster, HD Edition, "Xbox One X Enhanced" update, or a sequel which makes the original redundant, are Xcom, Bayonetta (unless you have a Switch), GTA IV and Halo: Reach (which is rumoured to be joining the Master Chief Collection shortly):
https://www.digitaltrends.com/gaming/best-xbox-360-games
I'd go seeking out the hidden gems, personally. The games that sold badly due to everyone and his dog buying CoD, GTA, and FIFA instead. The games that, looking back, reviewers were maybe a bit overly harsh toward. There's plenty of games in both categories. And the financial risk of picking a dud is <£5 now instead of ~£40.
For a few ideas:
Too Human
Lair
The Saboteur
Vanquish
MAG (not sure how many people still play this)
Heavenly Sword
Lost Odyssey
Enslaved: Odyssey to the West
Warhammer 40k: Space Marine
Deadly Premonition
Alpha Protocol
Dante's Inferno
Divinity II
Kingdoms of Amalur
Singularity
Spec Ops: The Line