Personally, I feel AlphaGo was the biggest deal ever. It put AI truly on the map. OpenAI is just a corollary to that, and would not exist without DeepMind in the first place.
What has OpenAI accomplished other than a lot more publicity for AI? If I've been following the story right, Google and a few others have created all the tech breakthroughs and OpenAI just created a way for common folk to play with it then sold out to Microsoft.
I'm not blaming DeepMind here.
It was Google's job not to start loosing ground to Microsoft in the age of AI.