What's hardest to learn and most lucrative?

I think sales skills are the hardest for people to learn because the first thing they have to realize is that they need to learn them.

I never took discrete math but I remember my bf at the time struggling through it. He was a pretty smart guy but then again I had to do all his C programming lab homework. Great computer engineer, terrible code monkey.

Music theory. I tried for a bit, then tried for a bit more, but eventually decided to toss it into my mental 'never gonna try again' bin. Now, I'm happy again :)

TopRatedTroll my degree is in mathematics but I always intended to be a programmer and discrete math was easy to me because a lot of it deals with CSci concepts. But a lot of the rest of it was way over my head.

josefinathegreat Voice acting and audiobook narrating like Simon Vance. Boy, does he make those characters come alive! Using a variety of timbres, accents, and dialects for male and female voices.

I don't think any skill is harder to learn than others. It all depends on what comes easy to you.

But there are some skills that all freelancers need to have, and that's sales (as @Mark said), marketing, negotiation skills, and being able to manage both yourself and your clients. If you can't do those things, then you'll have a hard time succeeding no matter what your niche is.