Of all gaming markets the PC is the hardest to define or track ...
When you factor in the massive quantity of flash games (as well as other forms of free games) the PC is probably the biggest gaming system by a wide margin, but when you're only looking at the sales of games through retail the PC is probably the smallest gaming system. From my understanding, PC games (generally) sell far less copies than similar console games but the lower overhead, alternate revenue resources, and the sale of expansions/mods generally make it a very profitable market.
From a graphical quality standpoint, the PC will always have the best graphics because there will always be far more powerful hardware avaialble if cost and heat are not considerations ... The typical PC gamer will not have access to this hardware and will scale back the quality of their graphics to see decent performance though.
