Before the carnival of stupid arrives to this thread, as it does to every Valve thread, I want to say that Valve's contribution to modern gaming in general and PC gaming in particular is enormous. Almost every single Valve title has been hugely influencial and almost all of them have secured a permanent spot in "best of all time" lists everywhere. Even if Valve never makes another game it will be forever part of any discussion on the greatest development studios of all time.
Yet for me, as a PC gamer, Valve's greatest contribution to gaming isn't its games. Steam revitalized PC gaming and I feel that we are all very lucky as gamers that the company that took a leadership position in PC gaming was Valve and not someone like EA, Activision, Ubisoft or Microsoft. While other developers and publishers were ready to write an obituary for PC gaming, Valve invested in that space and created a modern PC gaming market that is flourishing beyond any expectation. They made PC gaming more accessible to the masses, gave developers a more direct way to reach their audience and provided gamers with a viable alternative to the walled gardens and online paywalls of console gaming. Valve also revitalized Linux gaming, created arguably the best VR system out there, made PC gaming more couch-friendly through Big Picture and the Steam Controller and in their spare time released two gaming juggernauts in CS:GO and DOTA 2.
I, like many of you, am disappointed that Valve isn't making the story-based single-player games that I love anymore, yet that fact can't erase the rest of Valve's stellar work.