Everything, as owner — not viewer. Your ad accounts, analytics, tag manager, domain registrar, website hosting/CMS, business profile, and social pages should all live in accounts you control, with the agency added as a manager who can be removed in one click.
The litmus question for any vendor: 'If we part ways tomorrow, what do I lose?' The only acceptable answer is 'our labor' — not your data, your ad history, your pixel, or your domain.
This isn't paranoia; it's the industry's dirtiest lock-in pattern. Agencies that run everything in their own accounts own your marketing history — and re-renting it back to you is the business model. Audit yours this week; our ownership checklist makes it a 15-minute exercise.