Nintendo, Blizzard or Bioware for me...
All have introduced some fairly unforgettable characters, franchises and stories.
Without these guys we'd have no BG, Warcraft or Zelda, imo some of the best games series' ever concieved.
Who can seriously imagine a gaming world without the effect that these 3 companies games have had on other similar games; Zelda has bought controls still mimicked by other games along with utterly memorable music, and characters that can be remade many times, yet remain utterly timeless, Mario has brought instantly recognisable platforming perfection, Pokemon brought the first game where people genuinely care for thier own characters, rather than the more generic caring for the main protagonists of a story, Warcraft brought MMORGs and RTS to a wider audience than ever before, as well as introduced us to the world of Azeroth, and Baldur's Gate and KOTOR brought us some of the most refined, detailed and enjoyable RPGs of recent years, that are still considered almost the holy grail for developers to look to.
All are great in different ways, but these 3 companies I'd argue have done for more than any other company, although companies like Konami come close, but just arent quite in the same league.