Amelia

Amelia booking is definitely the best booking plugin for managing appointments. It's very flexible and also provide you with the required tools to implement a professional front-end booking system. I've literally tried almost all the competitors out there, and if you out-source your "employers" it provides an easy front-end dashboard for them to manage their services and bookings.

The support is helpful, and every month there are new features/functionality that comes out. I was very glad when I could get it on AppSumo :)