The choice of the RTOS strongly depends on the application domain. In each application domain (e.g., avionics, automotive, military) there is a leader. Also, because each application domain has different certification rules (e.g., OSEK/AUTOSAR in automotive, DO-178B in avionics, etc.) and some RTOSs have not been certified for all the standards.
In my experience, WindRiver VxWorks is the most used across all application domains.
But I know companies using GreenHills Integrity (avionics), Windows CE 6 (consumer), LynxOS (military) Vector and Erika Enterprise (automotive).
