Finishing it..... and beginning it. Its the part in the middle that's bad.
Either line art or thumbnailing. Admittedly, my favorite overall is the behind the scenes work. Writing, concept art, etc.
I love creating insane action scenes when Lyza is fighting a villain.
Something like that, it is challenging but fun. It's also fun when you have your main character ram a fist into a villain's face. It feels so satisfying.