What is the most challenging subject in math?

[fourier jr]

Everyone seems to have their preferences, so I wonder what people prefer the least, or what they find to be the most difficult subject in math, sunject you could never get your head around etc etc? (I know it's different for everybody, that's why I'm asking)

 

[nonequilibrium]

I personally find nothing as hard as Algebra (referring to the theory of groups and such), because it's simply so abstract you cannot even form the faintest intuitive idea, something you can do with Hilbert spaces, projective geometry, etc ...
It might also be the style in which I've received it: a heap of definitions and then theorems logically derived from these definitions with proofs that are logically correct but have no intuitive thought process behind them (or at least don't seem to...) with no story line to tie the different definitions and theorems together.