I also think Act 5 Act 2 is hard to beat, and I don't say that to knock any other part of the story. In addition to the epic final crisis of the Beta Kids session and having all 12 Alternian trolls in play, it also has some really good metanarrative stuff. That whole "scratched disc" sequence where Doc Scratch is narrating key events from both main plotlines from his hideout, whilst different scenes play out in the frickin' top banner? The epitome of "This really shouldn't work, but it so does."

Plus, Cascade. Just Cascade.

(I believe some of my favourite bits of the Act 6 complex are technically intermissions? Like "Game Over" and the dream bubble stuff.)