I have taken all of these classes and all the exams. This, however is just personal experience.
Easiest to Hardest Classes:
1. AP English Language
2. AP Environmental Science (Mainly because I've already taken AP Bio)
3. AP US History
4. AP Chemistry
Easiest To Hardest Tests:
1. AP Environmental Science
2. AP US History
3. AP English Language
4. AP Chemistry
Scores I received.
AP Environmental Science - 5
AP English Language - 5
AP US History - 5
AP Chemistry - 3
AP US History is the most popular AP test out there, and chances are you'll already know alot of the multiple choice answers just from common knowlegde if you're a good student (before you even take the class). If you do good in the class and dedicate half the time you think you will at the beginning of the year, I think you'll be fine. I honestly thought the test was really easy.
AP Environmental Science was really easy for me because I had taken AP Bio the year before, studied my butt off and recieved a 5 on the test. My Bio textbook covered alot of the relationship theories covered in the Environmental Science plan. Obviously this was the easiest for me because of my background, but overall, I think the test (and class) was pretty easy. I breezed through the test.
AP English Language, the class, was really easy because my teacher didn't know what she was talking about. It was her second year teaching the class and out of the 26 kids that took it the first year of her teaching, only one kid took the actual test. He got a 5...and then went on to the University of Pennsylvania. The test, therefore was alot harder than what I was expecting for the majority of the year. Luckily I got two study guides, freaked out a month before the test, studied the hardest I had ever for an AP exam and got a 5.
AP Chemistry is hard because I don't understand the basic mathmatical and scientific laws of Chemistry. I got an A in the class by cramming, memorizing, and manipulating my calculator, but retained very little info by the time the exam rolled around. I prayed, but basically gave up two weeks before the exam. I memorized basic conversions and survived the exam.
As far as study guides, I would honestly listen to your teacher above all, especially if she has experience teaching the class. Alot of teachers will receive actual study information from CollegeBoard, so put aside the study guides until you know everything your teacher tells you. But in terms of books, it really depends. I used Barron's for Environmental Science, Princeton for English Language, Princeton & 5 Steps to a 5 for US History and bought Barrons and Princeton for Chem, but didn't even get half way through Barrons.
Also, I would reccommend avoiding the latest versions of these books, as new information finds their way to study guides much faster than the actual tests.