Old fogey time!
You know how you can't have any discussion about the greatest bands in rock without some greybead bringing up how none of your favorite bands would even exist without The Beatles or The Rolling Stones or Pink Floyd? Well, none of your CRPGs and their huge, open worlds would exist without Ultima. I can't think of a more venerable or memorable world than Britannia.
It has stayed more or less geographically consistent from Ultima IV all the way through Ultima's V, VI, VII, and the execrable IX, as well as both Ultima Underworld games and the current Ultima Online. That's 26 years, to anyone who wants to count. You rescue it from cataclysms and evil overlords, from religious cults and otherworldly invaders, from despots and the denizens of the underworld, and most specifically from itself...from its own racism, greed, cowardice and spiritual bankruptcy. This is a world and a gaming series that brought us day and night cycles, fully interactive environments (you could fire a cannon, or bake a loaf of bread) and NPCs with their own daily routines independent of your actions two decades before Oblivion's radiant AI was a twinkle in a developer's eye.
From Mongbats to Moongates, I'm not sure there's ever been a virtual world that has impressed itself upon me more. I'd like to give a special shout out to EA for buying Origin Systems and running the series
into the fucking ground I'll hate you forever EA rowrowolrwowlrwowlwroloooo...