The standard response is AMD, but both their open source drivers and proprietary drivers are very bad. It's not performance, it's stability. AMD Linux drivers crash on all sorts of things. Having an accelerated desktop can do it.
I had a 7770 for a year before I switched to Nvidia. Better all around.
Still AMD do sponsor open source driver development, I'm told, so it's up to how much you're willing to sacrifice for this.
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.
> Intel is ok too, but their hardware is not really targeted for gamers.
FWIW, I find the latest graphics units from Intel, e.g. HD 4600, rather okay. Even graphically nontrivial things like Crusader Kings II or KSP run decently, although usually far from the highest possible settings. But on the other hand, the drivers are rock-solid, delivered directly in Debian without any fiddling around as was/is necessary for the Nvidia drivers and I don’t need my own powerplant just to run a computer :)
I don’t need my own powerplant just to run a computer
The Maxwell-based 750ti is the card to buy then. It's tiny, draws a reasonable amount of power and I've yet to see a game that doesn't run well on high.
I don't really game any more but like to support the ecosystem.