I think there's no universal harder subject in math. (Some people can find very easy what most people find hard, and vice-versa)
That said, if this is of any help I show you the Topology's assignments my University gives to math majors :
topología (click on the clickeable links of the page). Unfortunately it's all in Spanish, but I don't think it differs that much from English. There are also some online translator if you're really interested in the page.