Since Google is the one that makes the OS, the phone that will give you the "best" experience, would be also made by Google. Hence, Pixel 2 or Pixel 2 XL from the current line. Or Pixel 3 if you want to wait till OCT.
That said, there are plenty of people here that use iPhones with the TAG Heuer Connected watches. Search these threads, as there are plenty of posts containing tips on how to resolve connectivity issues.
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