Any of the foreign Language/Literature Tests: The class will help you speak and do activities in that target language that you cannot do at home (unless of course you're a native speaker

)
Calculus AB/BC
Physics B/C
Unless you're generally good at math/science, and able to teach yourself the concepts without the aid of another person than self-study the above, but a class make the above test MUCH MUCH MUCH EASIER. I self studies AP Physics and got a 5, but it took a LOT of work
History: Its just knowing the facts, but sometimes the class helps in digestion of the information, but overall they're just about knowing facts, etc. The dbq can be easily learned from a review book.
Govt./Econ: Not sure, I guess the same thing applies as History.
Lang/Lit: You're probably required to take these in school anyway, but the class doesn't really help at all...
MY PERSONAL FAVORITE:
Computer Science A/AB: I self studied and 5'ed both exams, but I had to do A LOT LOT LOT of programming in my free time. I also bought a textbook and went through it

My advice would be to just take the class for this, the concepts can get tough sometimes, and debugging code is a *****
Hope this *somewhat* helps!!!