First of all, I disagree with IndianJatt. I still say Bio. Bio is a very difficult class for most because of the sheer amount of information you have to take in, allowing yourself to do some of that in high school will greatly prepare you for college. Furthermore, for me, concepts in chemistry are easy to grasp, and it sort of is like a math class (you have to take in a lot less information, and complete repetitive problems/tasks). Lastly, there is an 30% overlap between bio and chem. Taking bio will allow you to grasp basic chemistry + biological concepts, so that in college you have a head start on the theory behind what you are studying, and all you have left to learn is the 'how to' (math based calculations in chemistry), or application of the science.
On a side note, World Lit. is the hardest AP class I have taken. For me personally:
World Lit = Bio + Chem + US Gov + Econ
I found Lit and Composition easy enough, but poetry is not my cup of tea. Combine that with mind-numbingly boring passages that you have to analyze and identify literary devices for. Personally, if I could reschedule this class, I would do so in a heartbeat. Lol, I still managed to pull off a B+ in this class though.