Surely PC is objectively the correct response, right?
I think it's cheating to count the whole 32 year history of IBM/Intel/Microsoft computers as one platform. You really have several different software platforms from DOS to 16 and 32 bit versions of Windows, and then 32 and 64 bit versions of Windows NT, which we are on now. There were also several major technology shifts as sound cards, then CD-ROM, then video cards were added to the platform. The transitions are hallmarked by very good backwards compatibility so the lines between them are blurred, but as you buy new hardware to play new games you will, over time, migrate to a new platform.
With that said, the Windows 9X period from about 1995 to 2002 has the best library. It contains the golden age of twitch FPS gaming, Infinity Engine RPGs, space/flight simulators, real time strategy games and turn based strategy games.