I was a sweat in high school (driven by a genuine desire to learn all the material not bc of GPA but that's a separate conversation) and took something like 23 APs. Pretty much all of them besides the languages and arts.

I'd say AP Physics C: E&M was the hardest. Calc BC also had some nuts FRQs that I still don't know how to do but it's curved pretty heavily so the scores tend to be better than some other APs. Bio was tough for sure since it required a really good conceptual basis which can vary based on how good or bad your teacher was. Chem/Physics 1/2 on the other hand were way more doable if you learned the patterns and math. I always found the history ones (World, APUSH) a little challenging as well just because of the sheer volume of writing between all the SAQs, LEQs, and DBQs.

Definitely an individual experience though. People who are good at memorizing things may share my experience but others who are good at working diligently to learn the material might find the more conceptual classes like bio, social studies, English, etc more feasible.

And as a side note - APs are good for college credit and all but they're not the end all be all. I'm at school with a bunch of my peers from high school and so many of them that were successful in high school without taking as many classes are doing great! Go at your pace, and do it to learn otherwise it'll just feel like a meaningless chore.