I think that this is such a tough question to definitively answer because the measure of difficulty of a problem is all relative to the particular point in time.

I am currently working on my thesis in artificial intelligence which to me seems tough because I have never written a thesis before. However, at work, I am dealing with technical software engineering problems that will seem easy after I have solved them.

My first industry project involved creating a generic form builder which could ultimately be used as a survey tool to draw statistics from. This seemed extremely challenging at the time, but now that all of the design decisions have been made, and complexities solved, I could redo it pretty easily (even though we shouldn't recreate the wheel)

Good thought provoking question though! Thanks!