Devices

Wear what you already wear.

Anything with a heart-rate sensor and a published API. Connections are read-only and go through each maker's official authorisation screen; you can revoke from their side at any time.

Not on the list?

If it writes active calories and heart rate to Apple Health or Health Connect, it works today through that bridge: Coros, Suunto, Amazfit, Withings and most others do.

Which is best?

Any of them. Scoring uses your device's own active-calorie figure. Chest straps and watches with optical heart rate score the same workout within a few percent of each other.