Archavon is probably the easiest. It's for people who weren't exactly raiders, and he is far, FAR easier than his cousins that moved in every start of an Arena Season.
He was a gear check boss. So his difficulty gets easier every new batch of content arrives. But during introductory content, he did lay waste to some poorly equipped tanks who had no other option for a few days.
Archavon however is consistently extremely easy to deal with.
Anub'Rekhan can fuck everything up if people literally have no idea what he does for the insect swarm thing, and they all stand there.