Well, I only have two arcs finished for now, with one currently going. But I'm still easily choosing the second one: Bad Night (chapters 6-8). Because I think is so much better than the first episodes which where mostly introductory in comparison.
The funny thing is, that it was supposed to be irrelevant to the plot at large. Something between the first arc and the next, much longer, one. To just be an entertaining arc where both main characters worked together for the first time.
But I ended up writing a bunch of events that are going to be extremely relevant to the rest of the story. In general, many important things are stablished and will affect many plot-points.
And it's also very fun to me and enjoyed writing it, specially the antagonist.