I5-i7.
I5-i7.
The top CPU choice for this setup is an i7 6700 or 7700, but the 8th generation models aren't compatible.
The compatibility was mentioned by someone else. Just because something fits doesn't guarantee it will function properly. There are two choices: either send back the CPU, as it might not be accepted due to its used status, or purchase a new compatible 300-series motherboard.
The issue is clear. 8000 and 9000 series Intel CPUs fit only with 300 series boards (like Z390, Z370, H310, etc). 6000 and 7000 models work on 100 series and 200 series boards (such as Z170, Z270, H270, H110, etc.). Even though they share the LGA 1151 socket, the older ones use version 1 while newer boards use version 2. It’s frustrating... The system you have is already set up; swapping a motherboard will be tough, especially if it doesn’t follow the standard ATX format. The best choice would be option #1 from Puckmyface. Consider getting the i7-8700K and a compatible replacement like i7-7700K, i7-7700, or i7-6700K/i7-6700. There are also i7-7700T and i7-6700T models, but they’re lower power and run a bit slower than the higher-end ones.