Air Law, not just because of the content, but because most people do it so early on in their training. With just a few hours under your belt, much of the detail bears little relevance to real life flying and so is difficult to remember.
Looking back on all the exams now, if I had to sit them all again, I'd say air law would be one of the easiest because you 'use' it every time you fly (the others to a lesser extent perhaps).
Mind you, still neither know nor care what year the Chicago convention was signed in
