My view? Such an agency doesn't exist, and if it does, please comment on this post with your secret.
From my experience, the one-stop agency hardly ever delivers one-stop service. They always drop the ball somewhere, have a weakness they don't know of, or don't admit too, and more often than not fail to offer a real holistic solution.
But one cannot blame the agencies. They need all the business they can get and will hardly ever say no. They know of the need for "full-service" agencies and often market themselves as such, despite it not being the full story. The reality is that marketing is much too broad, too complicated and too fast-moving for a single agency to be able to deliver everything.
Think about it: not even the global giants work with only one agency.
A core pillar of strategic marketing is this idea of an integrated marketing mix, meaning a combination of tools should be used together to achieve clearly defined communications objectives. Such a marketing mix should use a variety of tactics to communicate a single brand message and move prospects through the purchase funnel. The nature of such mix f tools almost always means using different suppliers to deliver the work.
Instead of looking for a single agency to meet with all your marketing needs, I suggest you instead focus on doing the right thing: define your marketing objectives and find the best partners to deliver on it in an integrated manner.
There is no such thing as the one-stop, full-service marketing agency. Effective marketing is like baking a cake - you need a handful of ingredients to make it tasty. Unfortunately you hardly ever get all these ingredients from a single shop.