Several apply, but most fans point to Beecher's revenge against Schillinger in episode seven, or Said rejecting Smug Snake Governor Devlin's politically-motivated pardon on television ("According to my religion there are some things I am not allowed to swallow!") as the most notable bits.
Beecher, high on PCP and having had enough of Schillinger's torture, smashing the window and sending a shard of Glass into Schillinger's eye. Both the prisoners and audience at home really couldn't help but cheer.
Although it has a terrifying subtext, Said protecting Beecher from the Aryans after his parole hearing is great. He literally appears from NOWHERE and shanks Schillinger and proceeds to do the same to Robson, and caps it off with one of the most chilling lines of the series.
Said: "Adebisi lives!"
Cyril punching out his dirtbag brother in season 3's "Out of Time" episode. Ryan's the only character who doesn't really get much comeuppance on the show, so the scene was double awesome.
As Cyril was being escorted to the electric chair, all the inmates of Em City casually stop what they're doing and enter their pods. While the guards expect the worse, they all begin to bang their hands on their pod glass showing their support for Cyril and Ryan. Think about it, the homeboys, Aryans, Latinos, bikers, Sicilians, Christians, Muslims, and non specifics coming together after trying to off each other for so long for the sake of two inmates.
What really makes it awesome is the individual shots of certain inmates:
Burr Redding still grieving for Augustus, Alverez and Guerra who hated each other for so long standing together, one of the homeboys loyal to Poet with no reason to care for the brothers, Busmalis and Rebadow showing that they still are as big hearted as they ever were, even Beecher who should've given up all hope and compassion by this point. For me it was the pinnacle of series, and a hope spot for all the failures of OZ and Em City.
And as an added bonus, this is shown to happen twice. After Cyril gets a stay of execution, but then loses his appeal and is executed again, none of the prisoners say "Ugh we already went through this." They just go back in their pods and bang on the doors again; this second scene also includes a prominent shot of Torquemada, who probably has never seen Cyril, joining in.
Martin Querns, just being himself. He's an amoral asshole, but one who can knock Adebisi's hat off his head, then stare him down afterwards without flinching.
Perhaps Bob Rebadow telling off Dave Brass should be something worthy of being mentioned: after Sean Murphy confesses his role in Enrique Morales' tendons getting cut, Brass tries to earn Rebadow's sympathy; the old man refuses and states that he has enough hatred in his heart for Brass and Morales already.
Beecher selling off rapist Adam Guenzel to the Aryans, for a well-deserved taste of his own medicine.