(I'm defining "user interface designer" as a visual designer plus interaction designer specializing in a particular platform, its HIG, and its toolkit(s).
That is, an experienced and professional enough UI designer is likely to have specialized in e.g. iOS, and asking them to design for e.g. Android will result in a potentially poorer UI than a similarly experienced Android specialist.)
There are no "best" or even "good" user interface designers as defined by external measures.
There can only be "popular" user interface designers, because a "good" user interface designer designs the interface that is "good" (visually comprehensible, accessible, follows the HIG, breaks the HIG rules in useful ways) for a particular platform, for a particular application, to be used by a particular user. A "best" UI designer is a function of experience: having designed more apps and having more experience, they are likely to make fewer mistakes, and come up with more refined interactions, and more usable new interactions, in the same amount of time. "Best" may also characterize how they are able to interact with a development team, such as having better estimates or a more professional working style.
These designers know they did good work because they conducted, or at least received feedback from, their users in usability tests, comprehension tests, etc. They did not rely solely on whether their client or employer continued to pay them for their work. Almost no-one publishes their UI test results, so there's no way you can find someone just by looking around, or even asking around. So all you have to go by, publicly, is the people who talk the loudest about design.
The only way to find a "good" or "best" UI designer is to look at well-designed applications on your platform that target your same users, and find out who did the design work. Don't look for names. Look for works, and then look for who made them.