You won't find the best designer on Upwork, TopTal or any similar platform

The best designers are busy being employed or in long-term contractual relationships with other clients.

The best designers get their leads through referrals and word of mouth. They are not active on talent platforms.

How to find a great designer:

Find a product you absolutely love (look and feel, function, UX, UI)
Figure out who designed it
Contact that person for recommendations of good designers who may be available for contract work.

The best usually recommend the best. You may not be able to get the designer behind your favorite product, but you can get a great recommendation from them.