My personal opinion: easy: stock maket corporate bond medium: governmental bond commodity future option stock market hard: fx spot swap/option fx index future / option What's your opinion?
Think whatever you been trading for years be easiest and whatever you seldom/never traded might be hardest, so think responses ae more geared to experience to some symbol than what is easy/hard.
Options. I still have no idea how they work...Maybe when I buildup a massive portfolio I will try to learn them.
...in conjunction with where that particular instrument happens to be sitting -- check the same instrument in 18-24 months, and it may be a very different beast.