I minored in math in college -- and majored in physics. Fortunately, my high school algebra and trig grades were high enough that I did not have to take college algebra. This was all in the USA, so unless the curiculums have changed drastically, this should apply.
Calculus 1 was Analytical Geometry and intro to Differentials. The geometry part was the hardest, I think, but no harder than trig in high school. Differentials were easy after I understood what they were all about. Using geometry to explain this stuff made it all "visual" and easy.
Calculus 2 was advanced Differentials and intro to Integration. Again, geometery was used to explain everything, and we learned that integration was the "converse" (or reverse) of differentiation. This was probably the easiest of all four semesters.
I aced the final without even trying.
Calculus 3 saw the introduction of infinite series and other tricks. This became not so easy because I could not visualize what was going on. More memory work, and the problems got trickier. But I made an A.
Calculus 4 saw the introduction of lots of things like partial derivatives, Calculus on vectors, Calculus on surfaces, gradients, infinitesimals, residual error. Some of these topics were no harder than Calc-2, some of them were very difficult, but overall, Calc-4 was only a little harder than Calc-3. Calc-4 was the last of my "easy" math classes. From there and on into graduate school, math required lots of sweat and total dedication.
