I'll just talk about Pokémon RP bc it's the only kind of RP I've been in so far, but worldbuilding might be my favorite part (also writing Jordie's battles, but that's more bc of the character themselves than because of anything directly tied to rping). But the part I enjoy the most is worldbuilding.
I guess it's because the pokémon franchise is so fucking dumb with its worldbuilding that trying to make sense after the smallest thing is really chaotic and fun to do (see: pokémon abilities), and doing it with almost the entire cast putting their grain of sand on how that stuff works makes it even better.
It's also because stuff has to make sense even if it takes place in a fictional world, y'know? So if the GM makes a cohesive/coherent world it's easier to write through the world and keep me invested imo. That's also the reason why I rarely take the anime as a reference, because at times they change shit whenever and however the script feels like.