Mine is naming things. I can hash out words all day, but if I have to give it a name, I'm at a loss. I wind up googling for ideas, searching wikipedia for appropriate references, researching all manner of things until something finally 'feels' right. I've finished my WIP and want to work on the query letter, but it HAS NO NAME. I had a name for it in the beginning, but it doesn't fit anymore. Plus it turned out to be the title of a cookbook already published.
Granted, this is nothing compared to the anxiety of naming my girls.
I know what we name them isn't important in the end. If a publisher picks it up, they usually change the title to something they think will work. But I can't just call it 'Beans and Rice' and be done. I can't send it out into the world like a nameless orphan begging for scraps on the corner. I over-think it, I know I do, but it's the one thing I'm anal about. The name HAS to fit.
What about you? What part is the hardest for you?