I started in TBC and went hardcore in ti and WoTLK, I remember the OMFG feeling when we downed M'uru and Kil'Jaeden
My main was warlock and I spent sooooooo fkin long to level the first time, and the mount quests omg, the mount quests ...
But TBC had one ENORMOUS problem, I had a friend that joined in when I was already downing Hujal and we simply couldn't raid together because there was no way in hell for him to catch up to me and later go into SW, WoTLK hit before that happened so he was stuck on Kara
So all in all my favorite time was WoTLK because it wasn't absurdly hardcore (note that hardcore does not mean difficult), it also had the best new zones for me, the best villain and the best music and lore (Storm Peaks is my fav zone)
I felt great getting "Alone in the darkness" from Yogg and walking around Dalaran with the Astral Walker title
I rolled holy pala for and remember auto atacking until I was 50 XD
But tbh my second fav xpack isn't TBC, it's MoP, if you exclude the Raid Finder the raiding is great, fun mechanics and a lot more is going kn, I also absolutely love the questing, music, Zones and designs and Lore of Pandaria