Why do you want to self-study an AP in the first place? I can only think of 3 ok-ish reasons (and a bunch of bad ones). The ok-ish reasons would be 1) you have a genuine interest in the subject (in which case you wouldn’t be asking which one, and you would be better off doing something practical in the field or taking a summer class in the subject); 2) in some way it would set you up for a specific job or internship (probably only ES); or 3) you are applying to colleges outside the US who use APs for admission (if so, watch out- some of them only accept APs from Jr or Sr year, and it’s better to have them focused on the subject you will be applying to study).
If you are trying to impress colleges by how motivated a student you are, don’t bother. They will be more impressed by higher actual grades and more depth in your ECs. If you are going for any of the AP awards, again, don’t bother- they won’t be at all impressed by that (same for NHS, btw).
Yeah, I decided I won’t self study any of them. I have come to realize after doing some more research that, it’s not about quantity, it’s about quality. Getting 5s on multiple easy AP tests isn’t the same as getting 5s on a few much more difficult ap tests.
I would add to @collegemom3717 that is you are trying to get more AP credits to be able to take less credits in college (so you have flexibility/to save money)
Good point @bopper - with the caveat that it can depend what college you go to- at many you can only transfer in a certain # and/or certain specific APs. But, it can make a difference.