The Elder Scrolls IV: Shivering Isles
Last year I called Oblivion my favorite game of 2006, and that opinion sticks. Good points can be made about the generic fantasy setting, reduced depth compared to Morrowind, and some promises less than completely fulfilled, like the new AI which had some silly problems. But I still enjoyed the hell out of it for a long time, and continued to do so long after I wrote that. Pretty recently I added a whole bunch of content, including most of the add-ons, the enjoyable Knights of the Nine quest line, scads of user-created stuff, and the expansion released last year, Shivering Isles. It all adds to an already huge game, and Isles in particular revitalized my interest in the game and its history.
Shivering Isles doesn't do that much to improve gameplay, there is some interesting additional weapons and magic, but it plays largely the same. What's great about it is how it steps up the enjoyability of the atmosphere. If you get into it, the Elder Scrolls series has some interesting lore and really intricate detail to it, although that's somewhat obscured by the blandness of Cyrodill, the province of Tamriel Oblivion mostly takes place in. That's not a problem with the expansion, which ships you over to the realm of Sheogorath, one of the other Daedric princes, who happens to be completely insane. Instead of lush hills and deciduous forests, the landscape is peppered with gigantic plants and mushrooms and all sorts of winding paths. Every resident is almost as crazy as Sheogorath in their own, unique way, and in contrast to Oblivion's pervasive seriousness, Shivering Isles is genuinely funny. It's always fun to meet new people and discover exactly what psychosis is bothering them that day. Sheogorath is the star attraction, and as far as talking heads that serve only to give you your next quest go, he's one of the most entertaining to listen to.
There's improvement in the amount of choice you have, too. Many quests in Oblivion gave you some choice about which way to go with a certain dilemma and the consequences varied, but those were mostly isolated incidents. There are a few choices you have in Isles that have real resonance with how the rest of the entire game plays out. It's still mostly the same, but it's different enough that your decisions seem to have an actual impact. In addition to all these design improvements, it's still an entertaining, rewarding RPG, and my only real complaint is that too many of the side quests are really just fetch quests, even if what you're fetching is more interesting than it usually is. There's still plenty I haven't seen, but I'd recommend it to anyone who has any interest in the series.