My favorite is thumbnails. It's quick (if you're following a script), makes me feel like I'm making significant progress, and gets me excited about the future story.
It used to be lines, but now I realized that my least favorite might be coloring. It takes the longest to do, especially if there's atypical lighting or tiny details. It's so annoying to think that I'm done only to find out that I forgot to color in the shirt buttons or put the shadow under the trees, etc.
I struggle with hands too, especially if they're positioned at a unique angle. They always somehow come out stiff and I still occasionally succumb to backwards hands syndrome. But I think feet is worse. I kinda regret designing barefoot characters.