I'd say probably AV, it taught me paying close attention to mechanics along with keeping hate and using a mark macro. Every other dungeon I could facetank through and rely on my healers. I think DD was the first place I really had to change targets mid combo to keep aggro. Had to relearn a bit with AK/WP, but now I can pretty much look at the healer's gear and guess just how many overpowers/flashes to use before settling into combo splitting so the DPS can tear the mobs to bits.