I have tried a number of tracking apps that have an Apple Watch app and like this one best. The watch interface is simple but complete, works well, gives good haptic feedback when stopping/starting, and I love that it also has a complication so I can just glance down to see where Im at. The interface on the phone is also straightforward to use. If I could have one more feature that I miss from some other tracking apps, it would be the option for a notification after a certain amount of time clocked in on a project (I would use it as kind of a built-in pomodoro timer).
Tfkempo about Hours Tracker: Time Tracking, v4.0.2