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Any of these:

* Garmin (required to adhere to GDPR)

* Empatica E4 (required to adhere to GDPR) (for more advanced use)

* Microsoft Band 2 (open source API on Github)

* Suunto (required to adhere to GDPR)

Personally, the Garmin Fenix 6 or Garmin Enduro (if the new Enduro gets good reviews) are the ones I would recommend for general use.

Apple may do various appealing things, but they are not obligated to adhere to GDPR. The app creators for the Apple Watch are certainly not required to adhere to GDPR.

Ironically, the ones that adhere to strict privacy regulations tend to have more solid technology. These technologies have been utilized for impressive research in journal articles.