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.