Healing is the most stressful imo. At higher end fights some of the damage being taken and the amount of healing needing to be pumped out at the right time to the right people can be insane. Tanking really isn't that bad, though it is harder than DPS. Once you get your initial aggro as a tank something needs to go pretty damn bad to lose it. I was a MT during SSC and MH both for short times and it really wasn't as bad as a thought it was going to be. The worst part was having to keep the raid moving and adjusting. DPS is obviously the least stressful,but I would mostly attribute that to the amazing amount of leaniency that they have on fights.