CentOS 7.5 is the latest stable version in the CentOS 7 series. You should use CentOS 7.5 (it's the version your CentOS 7 system would automatically update to anyway).
Keep in mind you shouldn't manually upgrade the server from CentOS 6 to CentOS 7 when cPanel is installed. You'll need to setup the new server with CentOS 7 and then migrate the accounts over to it using the guide here: