I can't think of a favorite off-hand, but my least favorite is research. It's crazy, though - I love doing the research. But I get too caught up in it and never get around to writing. Alternatively, I don't realize when I start writing that I'll need to research things, and then I try to write past it but it's a plot point, so I start looking it up and get distracted and never return to the story. Yeah. It's ridiculous.