This is a very interesting question. If i were to answer it in simple words, it would be designing parts of the game that are cool but if i were to answer it normally, well here we go:
The most fun i get when designing my project is the story, the characters and the brainstorming of ideas with my dev team.
It's just something so special to me when creating a story,i always wanted to tell a story in one way or another and upon realising that i could do that through my project, i decided to try it out and see what happens. I always get inspired by something, music, game or just a simple idea. It makes my head think a lot. And sometimes, i get crazy ideas that i am so thankful to have because it makes the whole story telling much more interesting and unpredictable for people who would want to play my project and that's the fun in it, i get to suprise people, i get to make them feel certain emotions through playing my levels, i get to make them think about my story, talk about it and want to engage in it. That's just so fun for me. I have changed a lot of my project so i can combine it my desired story in the forms of Story Arcs, a mean of progression instead of your normal nintendo worlds that we are so used to. Characters have a purpose, they are all distinct and they have personallity, villains and rivals as well. So to be honest, i have most fun when working on the story of my project. I haven't seen a story driven smbx episode before so some stuff are new to me but i have few people in my dev team who keep me in check and don't let me go off the rails with the story telling haha. I also have fun when designing certain levels or when i design bosses. I swear, designing bosses is one of my favorite things to do in smbx, i have made a lot of bosses and i keep making more because they are so fun to make, just the idea of having the player face against an enemy capable of posing a worthy challenge to the player and to engage in an epic battle is just cool. But yeah, i have the most fun when working on the story of my project because i really, really want to tell a good story that would make YOU want to get engaged in it and also when designing bosses and certain levels.
Also i have the least fun when i have to design levels which require me to put a lot of work in it and also when i have to create graphics.
Writing scripts and fixing bugs is also not really that fun. But what makes me really pissed off is when i have to polish a level 10000 times because it always has flaws in it. Well that's it from me, i will be looking what others have to share as well.