That list does not exist, and this is not modesty. Every ranking you find measures something other than what you need.
What the lists actually measure
- Award rankings measure how many prizes an agency won. That says something about creative ambition and about willingness to pay entry fees, and nothing about how they execute an annual plan.
- Revenue rankings measure size. The largest agency is the one with the most clients, not the best one for your client.
- Directories and "top ten" articles usually measure who paid to be listed, or who ranks best in search engines.
None of the three knows your assignment, your budget, your sector, your internal team or your way of deciding. And it is exactly those five that determine which agency works for you.
What "the best" really means
The best agency for you is the one that understands your assignment, puts the right people on it, and fits how you make decisions. That last point is almost always underestimated: an agency used to a single decision maker stalls in front of a steering committee of eight, however good the work.
What to do instead
Build a longlist of about six based on what you need: expertise, sector experience where it counts, a size that suits your budget, and financial stability. Invite three. That process takes longer than reading a list and produces an answer that is about you.
Which is why IKAg publishes no ranking but factsheets per agency, with the facts that can be checked: services, sectors, reference clients, awards and the figures from filed annual accounts. What counts as good is your call.
See also: how to find a good agency, how to know whether an agency is good and which agency fits your sector.