I thought it was the hardest in the Calculus sequence, by far.
I actually took it at the same time as Calc III (I was kind of learning Calc I at the same time as well) and Calc III was much easier and more intuitive...even though I was taking both at the same time. (So I was doing double and triple integrals at the same time as learning integration in Calc II.)
The other thing I've noticed is that I use the material from Calc II the LEAST out of any of the calculus sequence.
First of all, you rarely come across exotic integrals like the ones you learn in Calc II.
Second of all, if I do come across a difficult integral, I end up using other methods...like differentiating under the integral sign, using residue integration, etc.
Same thing with the methods we used for Series in Calc II...If I need to find out if a series converges, I pretty much only use the ratio test. Or, if I can't do that...there is some way to tweak the series by differentiating or integrating term by term, algebraically tweaking it into a better form, etc.
I hope I don't get bit somewhere down the line, but I really don't remember much from Calc II. lol
I've come across the need for partial fractions in Complex variables, but then you learn that if you have a simple pole, you can differentiate the denominator...and if you have a higher order pole, you differentiate the whole function, etc.
The more I think about it...I don't know how much trouble I'd have if I never even took the course.
(I'm sure there's important things I'm not realizing right now, so I'm really only half serious)