Real-time operating systems that I have come in contact with, in order of (subjective) impact:
Especially VxWorks has a long history in critical applications - for example, in cars and various NASA space platforms. It is however neither free nor open source software - I would probably prefer eCos or RTLinux in one of my own projects.
This Wikipedia article also has a section with what is supposedly a list of the most deployed RTOS, although its contents are not supported by any references.
That said, if you search around a bit you will find that the most deployed RTOS are usually found in proprietary embedded systems. Many of those actually push the definition of an Operating System quite a bit (e.g. the various exokernel designs out there).
Without more informaton on your use of this list, we cannot provide more information. If you intend to develop RT software, for example, there are far more factors to consider than the popularity of each RTOS. Cost, supported hardware, familiarity with the various interfaces, vendor support for specific applications, quality of the development tools etc. should all be taken into account...
