Windows XP made lots of money and is hugely popular...
For my part I'll go with Age of Emnpires II: Age of Kings, that has to be the single most balanced game ever developed, it seems like every other game nowadays they have so many stupid limits on what you can build and they always release patches to nerf the newest popular strategies, but AOK never needed patching and yet the strategies kept changing all the time since there really was no single strategy that had the upper hand.