College Board offers many Advanced Placement (AP) Math and Science courses, and it can be difficult to decide what courses to take and when. Here's a rundown of Math/Science APs and their levels of difficulty.
For reasons unknown, the College Board groups its 2 Computer Science courses in with the math courses:

AP Computer Science A

and AP Computer Science Principles.
There are only 3 AP Math courses offered:

AP Calculus AB,

AP Calculus BC,

and AP Statistics.
There are many more AP Sciences offered:

AP Biology,

AP Chemistry,

4 different AP Physics Courses,

and AP Environmental Science.
Across the country in 2018, more high schoolers took AP Calculus BC than any other AP course. This chart shows approximately how many students took each test.
If you are deciding how difficult a course is going to be, the most important thing to know is how strong the instruction is at your school. Find out by asking people who have taken (or are taking) the course.
Ask them how good the teacher is, what scores students normally get in their class, and whether the teacher’s tests are harder than the AP exam (this is a very good thing).
The sad truth is that there aren’t really any “easy” AP sciences. Whatever course you take is likely to be a challenge; the more your teacher challenges you throughout the year, the more likely you will be ready for the test!
Now, let’s take a look at 2019 AP Scores for all of the classes we’ve discussed so far. The AP Math & CS 2019 Pass Rates Chart compares the AP results from all of the AP maths and AP computer sciences. Notice that all of the AP Math classes have a “pass” rate (meaning students score 3 or higher) of significantly over 50 percent.
If there’s an AP math that you feel comfortable taking, you absolutely should! These classes are also very likely to be accepted by most colleges.
Despite a higher “pass” rate for AP Calc BC, it is undeniably harder than Calculus AB. It covers literally double the material in the same amount of time. Why is the pass rate so much higher? Students who take AP Calculus BC are typically very confident in their skills, that confidence and those skills convert into great results!
The AP Sciences are a little harder to assess.
There are two AP sciences in which more students who take it fail than pass: AP Physics 1 and AP Environmental Science are very hard in that specific sense.
If you are considering either, I’d urge you to investigate the quality of instruction at your school if scoring a 5 is a high priority. I’d also say that it isn’t necessary to score a 5 in order to get benefit from these courses. Many colleges will still accept a score of 3 or especially 4 as replacement credit or even credit towards graduation.
Most students who take Physics 1 are juniors, taking it as a preparatory course for Physics C is a great choice. AP Environmental Science integrates elements of biology, chemistry, and physics, making it a great primer for any of the above courses or virtually any STEM field in college.
Finally, I present my personal difficulty ratings, exhaustively researched and meticulously chosen: 1 is the hardest and 12 the easiest.

AP Physics 1  There are just so many reasons. You’ll take it earlier in your career, before the reality of college starts to be a motivator. It covers so many topics and is the foundation of a lifetime of Physics learning!

AP Environmental Science  There just aren’t a lot of people teaching at high school level with the expertise to teach this class. It covers so much amazing material! I love this course, but I can’t deny it is a hard class.

AP Physics C, Electricity and Magnetism  Despite the high pass rate, this class is hard because not many people even quality to take it. Plus, magnets man, how do they work. induced EMF may be one of the trickiest concepts taught at the high school level, period.

AP Physics C, Mechanics  All the hard things about the above, but less ridiculously complex topics. Sure, rotational motion can bring a half dozen simultaneous equations together, but there’s no magnets. Magnets, how do they work?

AP Biology  A lot of material to cover but less math based than many of the other courses. But it is so much material to memorize.

AP Chemistry  This one is hard, but there are a lot of great teachers out there whose mastery and passion of and for the topic makes it a safer bet!

AP Physics 2, Electricity and Magnetism  If you took AP Physics 1 and still want to take AP Physics 2 you know what you are getting yourself in to!

AP Calculus BC  Same as above. Calc BC taught by a great teacher can set the stage for a college career full of good grades in your math classes.

AP Statistics  Many choose to take this as a substitution for Calculus, but dang, it is tough in its own right. There are lots of formulas to memorize and prove master of.

AP Calc AB  Covers half the material of BC in half the time. It gives you a chance to really get your foundation strong.

AP Computer Science Principles  Data says few people pass this class than A.

AP Computer Science A  Coding is great, because every time you compile you automatically check your work.
Thanks for reading! Don’t forget, doing hard things is good for you!
Now go on out there and get a bunch of 5s! If you find that halfway through the year you could use some assistance with your course, we’re always here to help! [Be sure to check out our guide to AP Humanities courses too.]