My most favorite parts is plotting what's next in the canon. Whether the story is big or small, I just really love worldbuilding and odd scenarios. My early work (even the sentence structure) was very sporadic. But slowly, I've been able to get around that and start putting things together while keeping things fresh and exciting.
I also REALLY love dialogue. Jokes, insults, bad comments, or just oddball digs at my own writing or the characters I've cherished and created for about 16 years is just so much fun. It's like writing a variety sketch but the only audience watching are the characters you created that aren't involved in the story, or any friend you happen to share on the side!
The thing I least enjoy is something much of us can agree on... having an amazing idea, but it either turns out/sounds alot different than how you dreamed it, even if said same style was dreamed over and over and over again. It's just... UGH. I hate it because I had some really really good ideas. But then the style/sentencing just changed and gets elongated or silly and I have to cut it. If I could, I'd make a whole doc for Cut Content of my stories (or Deleted Scenes) just to show what originally was planned but just didn't work out.