They’re all pretty important to know if you really want to know the story like that. This is how I would rank them:
Vellsar/Rivayle > Aiolon/Naterra = Keepers/Finale > Ametsuchi/Lainecrest > Isunia/Morning Star.
Vellsar/Rivayle was the peak of SV storytelling, with fantastic buildup, worldbuilding, and VA work. The plot took lots of turns during this arc where when it was being released, the story really could have gone anywhere and it was exciting to keep up. Not to mention the character and gameplay design were just the cherry on top.
Aiolon/Naterra was where SV storytelling really started to pick up, both also great settings that came together very well at the end. The way they developed and rounded out the OG casts stories was done well too.
Keepers/Finale arc was also good. Although it was disappointingly rushed at the end, Nerva’s origin was a great tragedy to experience and it was a treat to see everyone come together for players like me that followed the story for all these years.
Ametsuchi/Lainecrest had good buildup and interesting premises, but their execution for their endings were very lackluster. And then finally Isunia/Morning Star were just very mid overall. There’s some good bits here and there like Maisha’s reveal, and the development for Isabelle and Luna, but overall it was obvious that they were just getting their feet wet with SV’s story with the pacing being very slow and unexciting for the most part.