Guess my list looks a bit like this:
1) Bioshock 1+2
Just awesome games, combining both an interesting and thought provoking story with awesome gameplay, great lvl design, and probably THE BEST atmosphere in a game i have seen in a long time. Also, who didn't shit their pants when they heard the first Big Daddy coming? i was pretty damn close
2) Simcity 4 (AND DON'T EVEN MENTION SOCIETIES! ruined the series

)
Boy have i used a lot of time on that game... There's just something about seeing your city go from farmlands to skyscrapers and hightec industries, and when you actually make a city bigger than the one you live in.. absolutely amazing game.
3) Portal
Again, a cool and thought provoking story, great wit, and some great challenges.
Other runner-ups include: Red Alert genre, for its awesome humor, Oblivion, mostly due to its huge moddability, and a heck of a lot other great games.