For me, I think it's setting, which to me is kinda the lamest, lol, especially if I were to write contemporary. Readers usually care most about plot and characters rather than setting; it's almost an after thought. Readers remember great characters and interesting plots and twists. Done correctly, readers don't think about setting because it should flow so naturally that they don't notice the world building. But having that be my one forte is lucky because I'm writing a fantasy novel, so setting is a BIG deal; I'm trying to create a completely different world so having a strong sense of setting really helps envelope the reader. Maybe that's why I chose to do a fantasy world in the first place.
As for characters, in my head I know what I want them to do, but they don't always come across that way on paper. I find myself revising and editing them a lot, trying to get the right voice and the right motivations for all of them. I think I get it eventually, because at the outlining stage, I usually have the clearest sense of the characters. Plot is definitely the hardest for me, which is ironic, because I like plot-driven books the best. I usually have a premise, but then finding a problem or something that needs solving, takes a lot of brain-storming. And then the plot holes! ugh. I don't think my brain works linearly, so it constantly goes off on plot tangents that I find myself having to reign in. Revising is definitely key. For now I think I'll stick with fantasy world building.
What do you think? Which part of writing are you the best at?