I'll have to contemplate this, although I'm not sure there's a clear winner for me. Oblivion is probably the game I've enjoyed the most from 2006, and it has its impact as well, but it's still quite flawed. One thing that's missing is Total War: Medieval II, which seems to generally rank very highly among the series, and it would probably be a strong contender for me as well, had I played it. And Dwarf Fortress came out as late as 2006!? Wow. Probably another strong contender... had I spent the time to learn it but alas, spending months to learn a game with a terrible UI (that's probably better now with the Steam release) hasn't seemed to appealing despite the appeal of the game itself.