AP Government - easy class, albeit boring unless you're into government. Extremely easy test too. Doesn't really require any skill to learn about government (unlike science or math AP tests), so it's great way to add another AP to your schedule without burdening your courseload. It'll get difficult, however, if you find government dull because then it's hard to take notes and learn.
AP Economics - there's two economic exams (Macro and Micro). Again, the same as government--it's interesting for some people but boring if you're not into economics. Slightly more difficult than government, but nothing majorly hard, especially if you have a good teacher.
AP English - take this. It's really hard if you're not a strong reader or writer. But a lot of colleges accept credit for AP English, so it's worth the difficulty. There's two AP English exams, and they're vastly different. AP Lit is about analyzing poetry and passages and other stuff -- the exam this year was insanely easy, given that you've been well-prepared for the good analysis and writing skills. AP Lang is about identifying types of writing styles in nonfiction -- like rhetoric, etc., and also requires analysis and writing skills.
Easiest is hands-down Government, unless you're an econ whiz or a literary master. They're all good classes for senior year, and in deciding which one to pick, I'd go with whichever one has the better teacher. If you know the AP English teacher is awful and can't teach at all, then don't take it. But if they're all taught by good teachers, then go for it!