Probably AMD, they fund open source driver development and release good documentation for their GPUs.

Intel is ok too, but their hardware is not really targeted for gamers.

That being said, if you are fine with using proprietary drivers, currently nothing beats nVidia in terms of performance, features and stability of OpenGl drivers.

You would be supporting the ecosystem with any purchase, but if you are politically inclined on free software then the choice is AMD or Intel.