Choosing an RTOS for the Cyclone V ARM Processor

"The best" is pretty subjective.

If your objective is to get some work done, as soon as possible, then I would argue that "the best" is the one with all the drivers you need already written and tested.

With no more research than a Google search I see VxWorks has been used ...

https://www.altera.com/support/support-resources/download/rtos_tools.html

And since that is an Intel company ... and so is Altera ... I'd say that VxWorks would be your best option if you want to use an RTOS.

The real question is why use an RTOS? You have an FPGA - ask yourself "Could all real-time tasks could be implemented in HDL, with a non-real-time-OS operating in a support capacity?". The answer I typically find is "Yes, I can just use the FPGA".

Of course this is system dependent ... but you should at least consider it ....

Cheers,

Dave