Fitbit (Google) Fitbit Charge 6

The best accuracy-per-euro in this category by a wide margin.

Fitbit rebuilt the optical heart-rate algorithm for this generation using machine learning and claims a 60% improvement in accuracy during vigorous activity — that is a manufacturer claim, but independent comparisons against a Polar H10 chest strap put the deviation at roughly 1%, which is the range you would accept from a research-grade device.

It carries a CE-marked ECG feature for atrial-fibrillation indication and an EDA sensor. And it does something almost nothing else at this price does: it pairs with a Bluetooth chest strap. When you want a measurement rather than an estimate — intervals, heavy weights, anything where the wrist moves — you can feed it real data.

Battery is quoted at up to 7 days, realistically 4–5 with the always-on display and GPS use. Built-in GPS is single-band, so it will drift in cities and under tree cover.

The honest limitations. Some Fitbit features sit behind Fitbit Premium, and the Daily Readiness Score is among them — you are buying a device with a subscription layer. The screen is small. And Fitbit's calorie estimate is subject to the same 27–93% error range as everything else here; the accuracy applies to heart rate, not to the number you probably look at most.