The whole "troll arc". Act 5 Act 1 and Act 5 Act 2 were really the peak for me in Homestuck.The characters and interactions between them kind of took the story to a new level for me from the previous four kids out of which I only liked Dave and Rose anyway.
I feel the same way. I think the story really hit its stride in A5A2, and was the best part of the comic. Sadly, it appears to be going through a lull in quality ever since Act 6 began. Everything after Cascade was a bit of a letdown, honestly, especially with what Andrew did with Dirk. That really angered me.