Android Wear is very open, it is very much the same api as normal Android. The main limitations are on the UI and a few specific issues like bluetooth advertising. Tbh, even Pebble didn't allow you so much access as Android Wear (e.g.: Pebble's bluetooth api was very limited).

Samsung's Tizen and Garmin's ANT seem quite open too, but I just studied the documentation, didn't actually wrote code for them.

Xiaomi's Mi band, FitBit and Apple Watch are very restricted.

I've started working on my take over the world side project and it has a watchOS component. I've been pleasantly surprised with the access to the CoreMotion and HealthKit data on the watch, as well as the ease of access for accessing the network directly or passing data and files back and forth with the iOS companion app.

Well, the future state will be a bit more unknown but pebbles are very easy to program and add do, and can be had pretty cheaply.