The best digital marketing agency is the one that actually gets you results. They have to understand your business, goals and be able to test to target the audience you need to get you the most results for your money/return on investment.
The best way to do this is test a digital marketing agency. Do not sign a long term contract. Give them enough time to ramp up and test campaigns, maybe a couple weeks to months, knowing you may not see any results. SEO can take 6-12 months before you start seeing results. Paid ads you should start seeing some results after a couple campaigns are ran over a couple weeks. Results could be new leads and traffic to getting actual sales.
Just because an agency worked for someone else, they may not be able to get the same results for you. It's all about minimizing your risk and testing different strategies from different agencies.
