Design agency covers at least four businesses that share a word and very little else. A brand studio makes identity. A digital product studio designs software. A web studio builds sites. A packaging studio designs physical goods. Asking for the best one produces a list sorted by marketing spend rather than by fit.
The four kinds of design agency, and what a UX designer agency is actually good at
Brand studios produce identity, naming, tone and the system that holds them together, and are usually weak at interaction. Digital product studios design software and think in flows and states, often with plainer visual work. Web studios build marketing sites and are strong on layout, content and conversion. Packaging studios work in physical constraints and regulated labelling. Very few firms are genuinely strong in two of these, whatever the capability page says.
Work out which problem you have first
If people do not know who you are, that is brand. If they know who you are and cannot use the product, that is product design. If they visit and do not enquire, that is web. If the product sits on a shelf beside competitors, that is packaging. Stating the problem in one sentence before you shortlist filters most of the market immediately and stops you buying a beautiful solution to a problem you do not have.
Best is not a property of a firm
Rankings and award lists reward memorable work for clients with large budgets and permissive brand rules, which may describe nothing about your situation. The firm that is right for you is one that has solved your specific problem, for a company of roughly your size, under roughly your constraints. Ask for two examples matching that description rather than for a portfolio, and be suspicious of a firm that cannot produce them.
What to require whichever kind you hire
Ownership of the files and the right to use the work without ongoing permission. A system rather than artefacts, so the work can be extended without the agency. For digital work, a named accessibility conformance level from the W3C's Web Content Accessibility Guidelines and evidence the designs meet it. And for anything claiming environmental benefit, awareness of the substantiation rules in 16 CFR part 260, which apply to marketing claims a designer may cheerfully put on a page or a pack.
Questions people ask about design agencies
What kind of design agency do we need?
State the problem in one sentence. If people do not know who you are, that is brand. If they cannot use the product, that is product design. If they visit and do not enquire, that is web. If it sits on a shelf, that is packaging.
Why not just hire the best design firm?
Best reflects memorable work for large budgets with permissive brand rules, which may describe nothing about your situation. Ask for two examples of your specific problem at roughly your size and constraints instead.
What should we require from any design agency?
Ownership of files with unrestricted use, a system rather than one-off artefacts so the work can be extended without them, and for digital work a named WCAG conformance level with evidence the designs meet it.