It all depends on how good your raid team is.
If your raid team is bad then healer is by far the hardest roll. You are constantly taxed on healing randoms taking damage which takes a toll on your personal focus. They won't use healthstones, they won't used defensive cooldowns, and they won't get out of the fire while they squeal about DPS/HPS charts.
If your raid team isn't bad then it probably goes to DPS since you tend to have to juggle threat, target switching, base rotations/AoE rotations/single target burn, interrupts, CC's, and personal focus. All this things prevent tank deaths, beat enrage timers, control mobs, and if they are worth a lick also heal themselves with stones and use defensives when necessary.
Tank over all is easiest simply because it is a roll that simply takes care of itself. Get mob, point mob this direction, use defensives when needed, and get adds. If you don't get heals it is someone else fault. If you don't get the mobs killed it is someone else fault.