World of Warcraft: The Burning Crusade
Mostly for nostalgic reasons, but as I recall it TBC is still the best expansion Blizzard made for WoW.
I had been playing WoW for quite some time before it hit. I was part of a really really great guild and had made tons of friends in the game. I still remember how the entire fucking server stood outside the gates to Outland and waited for the clock to strike midnight(?). The lag, the disconnections, the vast battle that ensued when alliance and horde had to stand together...
Then we got to the content. Vast zones filled with quests, lots of really intresting instances. We geared up together in the guild to be able to raid, which ment endless instance farming for rep and practise and damn did I have fun. I even spent(wasted) 24 hours of in game time to get exalted with Mag'har and The Consortium for a fucking dagger and mount, and I still enjoyed it.
Good times.
