Quite frankly the best PC RPG ever made.
The open-ended nature of this game is basically like Medieval Sims, you can do the main quest or not at all and can tailor your character in almost every conceivable way. The statistics system is definitely indepth and gives the game a lot of credibility and customization.
In addition to the lengthy campaign and subquests, the ideas of factions and various guilds and job types makes for a much more believable RPG than say WoW, where you are pretty much restricted to the class you set out to be. You can be a fighter, mage, and master thief all at the same time. This in addition to the ability to be a vampire and other minor factions truly brings the world of Tamriel to life.
The music and sound is awe inspiring and well worth releasing an OST all on itself. It is glamourous and truly befits the game's amazing setting.
Oblivion is definitely one of the best games of 2006. The only wish I have is that they have an MMORPG version of this ==> definitely will be the greatest game ever made.