hmmmmmm this is tough!!
Favourite part: Wrangling flow, timing, exposition, etc. in dialogue. The part where you know that twenty things need to be communicated here and also this joke, and you have to figure out how the conversation and events flow together to make it natural and reveal character organically. Making it feel less like you learned a bunch of facts and more like a character has opened up to you. It's super challenging sometimes, but it's the most rewarding feeling when you feel like it works.
Least Favourite: Setting things up. Fantasy by its nature requires SOOOO much extra establishing of concepts, plus the set-up demands of any story, where every idea you have for a scene requires you to remember to set up the relevant pieces beforehand so it's as impactful as you wanted it to be when you get to it. Setting up the things your character has already done/thought about/decided so it's not an asspull later when they announce it in a dramatic moment. All of these sorts of things feel like so much to juggle to me, and it's probably the biggest place I feel like I could use improvement when small things get missed or skipped and end up looking like small conveniences.