For me probably the headache that accompanies orthostatic intolerance and feels like a suffocation of the blood vessels leading to my head. Like an altitude sickness headache, I'd imagine. I hate this the most because it feels like if the back of my head is suffocated enough to hurt like that it is likely causing brain damage. It's not even such a sharp pain as a migraine but it is very heavy, and suffocating, sort of like there are lead bars up the back of the neck and into my head. It's a toss-up between that and the deadened, flattened feeling of my brain, but I think the suffocation headache has probably caused that, over the long term. Blech, going to read something more fun now!