Which video game series have left the biggest legacies? How have they?

[Nicklesbe]

Doom, Wolfenstein, Unreal tournament, Half-life. Those games made and defined FPS and good online FPS multiplayer. Everything today especially Halo and COD are imitations of those games and wouldn't exist without them. Then there are franchises like Elder-Scrolls and lets not forget D&D which are pretty much responsible for great game modding and well polished RPGs in general. Without D&D and other P&P greats I doubt any of the game developers would exist today.