The operating system you choose depends solely on the type of smartphone you have because the smart watch is paired with your smartphone. If you own an Android smartphone, your smart watch should have Android Wear or Tizen. If you own an iPhone, choose an Apple Watch. For example, the Samsung Gear S3 runs Tizen and is compatible with both the Android and iPhone.
Of the scores of Wear OS by Google (also called Wear OS, and formerly Android Wear) smartwatches that function nearly the same, the Fossil Q Explorist and Fossil Q Venture are the ones we recommend for most people. Similar models that differ mainly in size and design, these two Fossil watches offer style and band options for the widest range of tastes while performing as well as (or better than) anything else out there. They handle notifications and your responses without delay, provide all-day battery life, swiftly handle voice transcription and Google Assistant questions, and provide casual fitness tracking. Their buttons and screens are responsive. They look and feel like good watches, too, which is something we think most people want from a smartwatch at this point.
For deeper integration with your Android phone, you can give Google's Wear OS platform a try with Fossil's $275 Q Control touchscreen smartwatch. It lacks GPS and support for NFC payments, which is why the Gear Sport is a better fitness-focused smartwatch for Android users, but Fossil's Google Assistant integration and stylish design make it a solid contender. However, smartwatches with Qualcomm's new Snapdragon Wear 3100 processor start shipping in October, so you may want to hold off until we put them to the test.