Ask HN: Which smart watch has the best metrics available?

I have an idea for a smart watch app that I'd like to develop but first want to capture biometrics from myself for testing the idea. I looked into some development APIs for some smart watches but was a little disappointed with some that they only capture or expose a very limited set of biometrics (ie. maybe only pulse).

Are there any suggestions on which smart watch is the best for developers that provides the best access to data?