Have to give another vote to Mass Effect here. Crowbar’s right, the universe is so accessible, but intriguing at the same time. Bioware really outdid themselves with the world building on this one. The level of complexity and depth to it is staggering and the games themselves have been phenomenally well realized.
Ratchet and Clank
Neverwinter Nights (More the campaign building tools. A lot of the fan made modules are incredible. No other game I can think of invites the kind of fan creativity that this series does.)
God of War
Also, KOTOR is the only really good thing that Star Wars resulted in over the last 10 years. If that counts than add that in there to.
I must confess a certain fondness for Assassin’s Creed IP. The first one was a bit weird and abrupt, but I’m absolutely loving the whole “Da Vinci Code” feel of the second one, which in retrospect adds a whole new depth to the first.
Also, I like stabbing people in the eyesocket :).
Portal was my second thought. While the world wasn’t as good, it was the most purely entertaining game I’ve played in forever, and there’s a key sequence in which the game changes radically, and which did the best job a game has ever done of blurring the line between deciding-as-a-gamer and deciding-as-a-character (I don’t know how else to put it without spoiling this utter gem of a moment).
I’m currently playing through the first Mass Effect for the first time, and I gotta say, I’m loving the hell out of it. The equipment and character levelling are actually not so much fun to me compared to other systems (e.g., Dragon Age, Baldur’s Gate, WoW, Diablo), but surprisingly, I don’t care so much about that: I love the dialogue, and I love the feel of the cosmos, and I love nerding out over reading the geological descriptions of every stinkin little planet I come to.
The thing is, Bioshock doesn’t have a lot of room for sequels (I think a prequel or two would be quite possible, though) and Deus Ex apparently has a third game coming, nothing’s been heard of it in two years so I’ll believe it when I see it.