There are pros and cons for both options:
A single master key for your Ansible account would reduce complexity but you would need to ensure the private key is very secure, even if the key were password protected you would want to ensure the key is encrypted at rest with a tool like BlackBox and backed up somewhere.
System unique SSH keys would help with security but it's preferable to have a tool like HashiCorp Vault to help manage access to those keys. This shifts the security burden onto your secrets server but the added benefits could offset the extra work involved with setting it up and locking down access.
Another way you can go about this is to have a master Ansible ssh key but prevent root level access on the account and use something like PowerBroker to control and log privilege escalation from Ansible.