Civilization II, Grand Theft Auto 3, and Minecraft are the three games that altered my perception about what video games are, and I spent a crazy amount of time with all three. Civ2 dominated my game time during my early teen years, I spent many sunny Saturday afternoons indoors, destroying those blasted Carthaginians. I enjoyed Civs 3&4, but they didn't have that same magic for me. In a strange way, as much as I loved Civ2, I was hoping the sequels would be a radical departure and do something different. But they both felt like tame upgrades to me. They were great games in their own right, I put hundreds of hours into both, but they didn't excite me.
Civ5 dared to shake things up and try something new, which resulted in it being a buggy mess containing poorly-implemented features at launch. But it was the first Civilization game that's potential excited me since Civ2. Nobody can claim it's a perfect game with a straight face, not even after 2 expansion packs, and I do favour some of the features from earlier games. But I can say that Civ5 is now my favourite Civ game to actually play (although Civ2 will forever be my sentimental favourite).