For this writer anyway, from easiest, to hardest:
comics
scripts (television)
scripts (film)
novellas
short stories
novels
personal essay
poetry
Not on the list: introductions. They’re generally difficult to write, but on the other hand, the pay is lousy.
This doesn’t say anything about what’s most satisfying to write, or what I take the most pride in. Perhaps because novels are the most challenging, they’re often what I feel most attached to, after the fact. But that can’t be a surprise — you live with a novel much longer than you live with, say, the script for a 22 page comic. Authors are always saying writing a novel is like having children. Actually, I’m pretty sure nine months of carrying a gestating creature around in your abdomen, and then ejecting it after ten hours of groaning labor, in a spray of H.R. Geiger fluids, is fucking noooooothing like playing make believe in your office for a couple hours every day. Still, maybe they are a little like pets. Sometimes they scratch, they take months to housebreak, they HATE baths (rewrites)… and some days they curl up into your lap and begin to purr and you feel awash in contentment. You can get attached to that last feeling. That can make all the hard parts worth it.