In current WoW, TBC is the closest thing to Vanilla WoW right now... in fact it is the most vanilla content in current WoW. Think about that for a second :P
But in my honest opinion TBC and WotLK are like the height of my times in WoW. At the beginning of TBC I got into a hardcore raiding guild (still in to this day) for he first time, I had raided in Vanila but we only got to BWL. I hold TBC in such high regard, so many good memories.