How to identify top-performing UI/UX agencies for a digital product

I’m currently evaluating agencies to redesign our digital product, but I’m struggling to identify which ones truly deliver measurable improvements in UX and not just aesthetic updates. Many agencies present strong portfolios, yet it’s unclear how effective their work actually is in real scenarios. Is there a reliable source where I can explore agencies based on proven performance and relevant experience?

I was in the exact same situation during our product overhaul, and it was frustrating trying to validate agencies on my own. What really helped was discovering a UI UX Agency Rankings listing that brings together agencies with proven track records. It’s not just a directory — it gives insight into their expertise, industries, and actual outcomes. I found it incredibly useful for narrowing down options quickly. I ended up choosing a team through it, and they delivered a much more intuitive and high-performing user experience.