1. TBC
Quests, dungeons, raids all tied together in a nice flow and created immersive character progression.
2. Vanilla
Huge world, beautiful zones and music, exploration, amazement at every turn.
3. WotLK
Beautiful zones, nice quests and 5-mans, heroics faceroll, Ulduar but Naxx recycled and ToC shitty, casualism took hold.
4. Cata
Class homogenization, healer imbalance, more recycled content, no continent, zones were good but not great.
5. MOP
Raiding overtaken by LFR, faceroll heroics, recycled heroics, mediocre zones, KungFu panda and pokemon, no lore just copy/paste Asian themes