Most compatible iPhone model for the Apple Watch is...
The motion sensing scheme where you have to roll your forearm to turn on the watch face is ridiculously bad and seems to be getting worse. It requires your arm to be kind of parallel with respect to the ground before you roll the forearm for it to turn on.
Any iPhone 5 or newer will work well with the Apple Watch. If you want the best performance, the iPhone 6, 6 Plus, 6s, and 6s Plus, will work the best because they have the hardware that was designed to work with the Apple Watch when the phones were created.
It will obviously work the best with the newest devices but it uses Bluetooth and wifi which aren't new technologies so the differences should be minimal.
The newest hardware, but the difference should be minor because it uses the same Bluetooth and wifi technologies.
The Iphone 8 plus as it provides the largest screen and the fastest processor.
- The motion sensing scheme where you have to roll your forearm to turn on the watch face is ridiculously bad and seems to be getting worse. It requires your arm to be kind of parallel with respect to the ground before you roll the forearm for it to turn on.
- The biometrics are completely off (despite a lot of calibration in the health app). The step-counting is about as bad as what fitbit has. From a technical perspective, this is no better than what we had with the pedometers in the 90s (except they had more accuracy for the price).
- The watch apparently doesn't store data on its own at all. So if you forget the phone in your office while you take a walk or climb the stairs, nothing gets registered or recorded. This is a very weird requirement from a tech perspective.
- You have tell it what exercise you are engaged in. For example, if you go on an elliptical without going through the time sucking setup, the Watch won't register the workout even as a walk.
- The fitness/activity features and step tracking are easy to throw off by just waving your arms around.
- If you don't use the Watch and leave it on the charger, the body becomes warm. It's not hot, mind you, but it does concern me when electronic devices get warm.