If you want a bare RTOS that works fine and is easy to port, FreeRTOS is a good choice. It's GPL with a linking clause so can be used in a closed source product if you provide rtos sources.Some vendor bundle it with their BSP/SDK (NXP, ST) with various improvements (like low power tick timer in sleep mode).
On the other hand if you want a full framework you can go with other like chibiOS (full GPL, so no closed source afaik), zephyr (still very young and evolve quickly) or nuttx. The inconvenience of these frameworks is that you need to embed everything in their build system and use their drivers. So if you plan to use vendor SDK, it's not the perfect choice and can requires a lot of work.