My money would be on the snake. I was watching a program on anacondas and their strength and they put a meter on a duck (that it was about to eat) that could measure the psi and it turned out that this snake (it was like 20 feet long) averaged around 90 lbs psi for more than 15 minutes before it released its grip. It said that if it wrapped around you like that it would be the equivalent of having a 9000 lbs school bus setting on your chest 
. It only took a few seconds for it to completly wrap around its prey to so unless you saw it coming I think you would be totally screwed. Also on this program they talked about an adult male who had been attaced by a snake that was a little over 20 feet & it crushed almost every bone in his body (possibly every bone). Just recently I saw in a news article that a 13 ft python killed and ate a 6 foot alligator in the Everglades (I think the snake died after it ate it though).
Edit: Found some links about the alligator getting ate:
http://news.mongabay.com/2005/1005-gator.html
http://www.foxnews.com/story/0,2933,171358,00.html
If a 13 snake can do that to a 6 foot 4 inch alligator imagine what a 20+ foot anaconda could do to your average sized human.
. It only took a few seconds for it to completly wrap around its prey to so unless you saw it coming I think you would be totally screwed. Also on this program they talked about an adult male who had been attaced by a snake that was a little over 20 feet & it crushed almost every bone in his body (possibly every bone). Just recently I saw in a news article that a 13 ft python killed and ate a 6 foot alligator in the Everglades (I think the snake died after it ate it though).Edit: Found some links about the alligator getting ate:
http://news.mongabay.com/2005/1005-gator.html
http://www.foxnews.com/story/0,2933,171358,00.html
If a 13 snake can do that to a 6 foot 4 inch alligator imagine what a 20+ foot anaconda could do to your average sized human.