Personally i liked ToC - trash is a waste of good time in my eyes. I will go with the massively unpopular opinion though and say either Ulduar, ZG or AQ40 for me, neither of them i really enjoyed to the point i wanted to play them, while they all had some fun encounters the problem was most of the bosses just didn't push my buttons. Might have a chunk to do with the fact i was either in terribad guilds (ZG/AQ40) or barely playing (Ulduar) but i didn't feel it. On the other hand, i enjoyed ICC & BWL so it's probably a trade off, still say Neferian back in the day was one of the most fun encounters i've played.