I really liked Il Siracusano's story, and how I couldn't really tell who was allied with who until close to the end, and how it really fleshed out Lappland as a character.
However, my favourite event is probably Maria Nearl. It holds a very special place in my heart, as it was the first side story I read when I started last summer. It introduced me to my favourite character (the eponymous Maria Nearl), as well as being a tried and true tournament arc, with a ton of behind the scenes drama.
It's a toss up between Dorothy's Vision and Stultifera Navis.
Dorothy's Vision has the entire simulated world vs reality. At the end of the day Dorothy was really just trying to create an alternative world. With the revelation that those in the simulated world were truly volunteers it puts things into a good perspective to me. Dorothy being the creator and destroyer of her invention solidified Dorothy for me.
SN has all of the seaborn lore going for it and I feel that every time we get a seaborn chapter we are just left with more questions instead of answers. Also a huge Irene simp.
Both of these also lightly touch on the whole fake sky / extra terrestrial thing where we continue to get hints but never answers.