BioShock 2. It's great, but can't overhaul the genius of the first. The atmosphere is a lot darker and moody (understandable after 10 years of neglect), so the locale isn't as stunning. Didn't find the story as engaging or shocking as the first, but it still scores highly. Plasmids and weapons are largely the same, but the gene tonics seem to be an improvement. I liked little touches such as my security bots having personal names. The little sister ADAM gathering got repetitive after a while, but I enjoyed the strategy beforehand of placing trap rivets + mini-turrets, waiting for the splicer onslaught.
Multiplayer has/had bags of potential, but the connection issues and lag are a major drawback. Sometimes difficult to find a game, so clearly isn't as popular online as your major war FPS games.