WoTLK for me. It was when I really started to learn how to raid, understand rotation and stat priorities. We raided hardcore and where one of the top guilds on the server. I was one of the raid leaders and worked to develop strats. It was the peak of my WoW 'career'.
I still enjoyed every expansion since but none will ever compare to that time. Now I am playing Classic WotLK and nostalgia is hitting hard.