Wotlk was the perfect expansion for me. I was hooked from start because we got to fight Arthas at some point - the lore was fantastic and kept you listening instead of trying to skip past it. I loved the dungeons especially pit of saron, desperatly hoping for the quest item for the sword to drop.
Ulduar changed my views regarding raiding in WoW, it was a breath of fresh air - so was Icecrown Citadel, loved the mechanics and lore and fighting Arthas himself.
It was also my first time doing arena on my hunter which was somewhat balanced midways in the expansion, unlike now where some classes can burst you down in 3-4 secs.
Cataclysm still haunts me though - even though MOP is decent I can't help but feel that raids aren't as really as they used to be. It's like everything went to shit when the main developers went to Project Titan whatever that is.