Ask that question 100 times and you'll get 100 different answers. Lucasarts. Sierra. Bethesda. Valve. Westwood. Ask me 100 times, though, and I'll give you the same answer 100 times running: Origin.
See, while I applaud "specialist" developers - those like Valve who continually progress the one genre or series - to me, if you're going to talk about the greatest PC developer of all time, it needs to be one that has proven itself across multiple generations, genres and series. Why? Because if a developer can come top of the class in more than one kind of game, it shows that there's brilliance running right through the company, not just in the engine room or art department.
It's why a studio like Irrational, which has brought us everything from Freedom Force to System Shock 2 to BioShock is so revered. The same can be said of Bullfrog, which was able to jump from Theme Park to Magic Carpet to Syndicate without missing a beat.
While I have great respect for all the developers named above, though, none can match the standards set by Origin Systems.
Origin was established all the way back in 1983 by the creator of the Ultima series of role-playing games, Richard Garriott, along with his brother Robert, father Owen and a fourth partner, Chuck Bueche. The studio's first game was actually the third in the Ultima series, after the first two had been published by Sierra.
Based in Austin, Texas, Origin would over the next 21 years produce some of the most beloved and successful video games to ever grace the PC, like the famous Ultima series of role-playing games (including Ultima Online, the world's first "big time" massively multiplayer online game), the Wing Commander franchise (including the excellent Privateer spinoffs), the "Strike" line of theatrical flight sims and the two Crusader games.
Across these games and more, Origin released critically acclaimed first-person shooters, role-playing games, flight sims and space shooters, and at one stage or another was home to famed developers like Richard Garriott (Ultima), Chris Roberts (Wing Commander), Raph Koster (Everquest II), John Romero (Doom) and Warren Spector (Deus Ex).
Sadly, Origin is no more. On the back of a string of successes in the early 1990s, the developer was bought by Electronic Arts. Rising development costs for Origin's ambitious games, the cancellation of several key projects and the departure of key personnel like Garriott around the turn of millennium spelled doom for the developer, though, and by 2000 it had ceased to exist in any form other than as a support mechanism for Ultima Online.
Electronic Arts finally wound the studio up in 2004, and has sat on most of Origin's properties (with the exception of an awful Wing Commander game) ever since. In a poignant touch, the last man to be let go from the studio was producer Jeff Hillhouse, the first person hired by Origin back in 1983.
A sad fate for such a storied developer, but an even sadder one for a succession of franchises that dominated the marketplace for so many years and now find themselves locked in a basement somewhere underneath Electronic Arts HQ.
That's my pick, then. What's yours?