The Burning Crusade
The. Best. Expansion. Ever.
It just happens to be the expansion I first raided seriously in. It was also the expansion I loved the content the most. The raids, the zones, the boss fights, the dungeons, the attunements for both raids and dungeons – ahhh, all the good stuff. The guild I lived in for that entire expansion was where I met the first of my prolonged WoW-based friends.