I was only ever at Google Brain, so I don't have direct experience at the other places you mention, but my impression was that there was somewhat more research

I was only ever at Google Brain, so I don't have direct experience at the other places you mention, but my impression was that there was somewhat more research freedom at Google Brain, particularly for theory. My impression of DeepMind was that it's relatively regimented and research is done in a very top-down way. OpenAI seems to have more of a focus on reinforcement learning (that's certainly done at Google Brain, too, but Google Brain does a lot of other things, too). I don't really know too much about Facebook Research.