You. Always you. The account holds your spend history, conversion data, and audience learning — assets that compound over years and can't be exported. The agency works inside it via manager-account access, which you can revoke in one click.
The correct structure: the account is created under YOUR Google account with YOUR billing; the agency's manager account (MCC) links to it with standard access. Their link, your property.
If an agency insists on owning the account 'for convenience' or resists transferring one they created for you, that's not a technicality — it's the lock-in, showing itself early. Walk.