Molten Core. Spent ages with the guild getting enough people together and running 5-10 man dungeons to get everyone decently geared up. Enter and see the two Molten Giants (I think that's what they were called) standing there and our RL explaining that his friend who had been here before and given him some tips (seeing as there were no strategy guides out there) had told him those two are extremely difficult to get past.
We wiped twice before getting them down, and as we all crossed the bridge and had time to look around, it felt so damn epic.