Top, because I am paranoid about being on the bottom and having it collapse and crush me. Seriously, I can't sleep on the bottom out of fear.
OP: Clever, clever title. Me wonders how many won't read the OP and post something drastically different.
My brother did that to me, once; we were both home from college, and my grandmother happened to be in town, so she had my bedroom, and his room had a bunk bed, and he had always had the top bunk, so I just grabbed the bottom one when I headed to bed. I'd never realized that my younger and 30lbs heavier brother had a tendency to vault onto the top bunk, and on this one occasion, when he did, 50lbs of bed and matress, with 180lbs of chubby college kid came crashing down on my sleeping self.
Thankfully, I was sleeping, and face down at the time, so I was completely limp, and in a position to push up into the wreckage once I'd become awake enough to recognize what had happened. I was still pissed, though.