Original: Demon's Souls
Demon's Souls made hack-and-slash hard again, but Dark Souls made it cool. The follow-up by From Software was a spiritual successor to Demon's Souls, but for all their similarities it may as well have been a true sequel.
The combat, visual style, tone, and even one character (damn you, Patches) made it into the new game, but Dark Souls also changed the formula significantly. It featured a new story, new items and characters, and tons of new mechanics, of course, but the biggest change was in the environment.
Instead of compartmentalized levels that had to be challenged one by one, Dark Souls featured one big, interconnected world that twisted and turned in on itself like a beehive filled with demons and pain. Dark Souls 2, coming out next year, will continue down that road, and the series will finally get a true sequel; but we feel Dark Souls will always be our favorite.