Hmm, I'm deciding between being crushed slowly or being on fire. I'd guess I would have to go with fire, since you could probably find a way to pass out while being crushed.
Ever been burned? Of course you have, you quickly react, and find a way to cool down the burnt area, yet, it still has that weird burning sensation for a while. Now, imagine burning to death, that whole burning feeling all over your body, plus it doesn't help that the contents of the human body are highly flamable. I never thought that this would be the worst way to die until my AP Environmental Science teacher brought it to my attention.