The Xbox store updates thier best seller list every friday and during the last week Cuphead took the number 1 spot. Cuphead launched the same day as FIFA 18.
1. Cuphead
2. FIFA 18 Standard Edition
3. NBA 2K18
4. Destiny 2
5. Rocket League
6. Madden NFL 18
7. Plants vs Zombies GW2
8. Titanfall 2 Ultimate Edition
9. Mass Effect Andromeda Delux Edition
10. Minecraft
Also steamspy has Cuphead at 328k sold.