I'm really not a fan of any sort of pain, but the most regular and reliable pain for me is that of my back. I first threw it out at the age of 12, and have been regularly in pain since.
I can function with a sprained ankle. I can handle it when my knee slows me down. But when my back decides to go completely out, I'm reduced to helplessness. And I hate it.
Not only that, but back pain has cost me more missed work than illness. By a lot.
And I really dislike that.