From TBC to SL, which expansion were you most invested in?
It could be time investment, as you had the most drive to play a certain expansion and committed the most time to playing it. Was it because of the story? Game play? Game mechanics and/or balancing?
It could be an emotional investment, in terms of the story being told and seeing how it would play out. Were you attached to certain main characters? Did you want to see a specific antagonist finally be beaten?
Was it something else altogether?
Were you/are you invested in every single one of them... or have you never been invested in an expansion?
Personally, it was WotLK for me. Not saying this because WotLK Classic, although I am choosing to play that exclusively over retail due to the following: WotLK was the first expansion that I got to experience from beginning (leveling to 80) and to the end. I was young during vanilla WoW and did not play it "seriously" until the near-end of TBC, when I first got to 70. After catching the proverbial "WoW itch" there was only like 2 months until WotLK came out... and the rest was history.