I'm with Andrzej, tapping. I'm not a virtuosos at everything, but that's the hardest for me. I'm slowly learning how to hit the strings correctly, but I don't practice it enough to use it anywhere yet.
I need to work on my scales more also, because my fretting hand doesn't move REALLY fast yet, but I don't look at that as something that's "hard" it's just something that I need to practice. Everything else I'm good with. Given the right song and when the Spirit is high, I can do things that I can barely do in practice.