From the Chapter 1 class trial, the line which cemented The Reveal. For context, the group has established that one of the cameras, which Kaede and Shuichi set up to capture the mastermind, had the flash turned on in order to lure out the culprit. Having proven that Shuichi couldn't have done it, Kaede reveals the existence of the person who set the flash and created the trap.
Tenko: Do you know who the culprit is?! Kaede: Yep, I know. I've known for a long time. ... Because there was only one person able to set the flash on those cameras.
The very last line of Chapter 2, directed at Maki and revealing her true talent:
Kokichi: Right... miss Ultimate Assassin?
This line halfway through Chapter 4's trial, during which Kokichi, then the prime suspect, tells the others everything knows about the case, heightened by two things: firstly, the speaker makes a Nightmare Face during it. Secondly, it's spoken without a hint of irony or sarcasm. The speaker is dead serious, making the implications even worse.
Kokichi: Well then...the culprit is Gonta.
Chapter 4 also has this line from Gonta, which basically confirms he has complete amnesia of the virtual world:
Himiko: (referring to the port to plug the consciousness cord in) H-he couldn't have messed it up! I said it over and over! (holds up right hand) Right is the hand you hold your chopsticks in!
Gonta: (holding up his left hand) Oh, but Gonta hold chopsticks with this hand.
K1-B0's speech at the very end of Chapter 5 is this for two reasons: it heavily hints that K1-B0's now-broken ahoge was actually the source of his "inner voice", and it sets up the beginning of the end of the Killing Game at the start of Chapter 6.
From Chapter 6, the line which cemented the exact nature of this game's reveal:
Tsumugi (as Makoto): If what I said was the truthnote that she can only cosplay as fictional characters and has an allergic reaction if she tries to cosplay as a real person then Hope's Peak Academy was... Shuichi: It's fictional... It's all fictional?! Tsumugi (as Byakuya): The Biggest, Most Awful, Most Tragic Event in Human History never happened... Hope's Peak, the Future Foundation, and the Remnants of Despair do not exist... Tsumugi (as "Byakuya"): Because it's all fiction. None of it actually happened in the real world. Tsumugi (as Kyoko): All of those events took place in a fictional world known as Danganronpa.
Approaching the resolution of the final class trial, Shuichi drops an unexpected objection:
Keebo: Everything is gonna be ok! We'll definitely find Hop— Shuichi: What do you mean Hope?I reject that Hope!
At the very end of the final trial, after voting time and the students worry that abstaining from voting was pointless, because K1-B0 and the "mastermind" will both vote for each other.
Tsumugi: Oh, there's no need to worry about that. I didn't vote either.
