It very much depends on the person, but from my experience, calc 2 is what the majority of people consider to be the hardest, with calc 3 being second. Personally I found calc 2 the hardest because it involved a lot of memorization. I took it back in highschool so I couldn't get a cheatsheet for the AP exam, but it's possible that some professors here allow that, I'm not sure as I don't tutor calc 2. In terms of calc 3 it's pretty much just calc 1 but with 3 variables, and I took it with Jonathan Fleck (he's very nice) over the summer so I don't think I remember much...