In Madhouse the final confrontation with Sawney Beane. Cal just cuts loose with his gates and teleport spams enough to pretty much wipe the floor with Beane, who has spent pretty much the entire book kicking the ass of all comers all with a deranged smile on his face.
In Deathwish, there's Niko's utterly terrifying suicidal rampage when he finds Cal dead. In the Heroic BSOD to end all Heroic BSODs, he finds the lair of the Big Bad, slaughters every single one of his pet monsters without breaking a sweat when the entire core cast were barely enough for a fraction of them, and only stops when he gets to Oshossi because he wants to inflict as much pain as humanly imaginable on him, all the while in a blinding haze of shock, rage, and grief that he can only describe as 'white'. It is a very, very good thing Cal's body was just an illusion.
Cal and Niko's refusal to forgive Kalakos in Doubletake. He is not Easily Forgiven in the tradition of so many Disney movies - the exact opposite, in fact. The entire book is basically one long, brutal Calling the Old Man Out for abandoning his son. Of special note is Cal's conversation with him after Kalakos doesn't understand why his son stabbed him for preventing Niko from reaching Cal when he was almost certainly dead.
Cal: Nope. I don't think you were wrong. Keeping Nik safe is it. Number one priority for me. Kalakos: Then you see. Then why won't he? I want my son back. We were making progress and now this. If it had been you I'd have held back, with Niko already beyond reach and gone, you would've understood. Cal: Oh yeah, I'd understand. If you'd tried to hold me back from getting to Nik, no matter what the odds, I'd understand you'd be breathing through a bullet hole in your throat. I'm going to cut you a break because you did save Nik at least once. But I know you won't get it.You look like Nik, but you're not half as smart, and wanting to invest in emotion for your son now? Is 'too late' not in your vocabulary? But since you want to try....So listen, Kalakos. Concentrate. I'd do anything to save my brother. Anything. And he'd do the same for me. He has done the same. Think it through. He risks his life for mine, Then I see the mess he's gotten himself into while pulling my ass out of the fire, and I'm right back in after him. Same risk. Same mess. Then back it goes again. Vicious circle, I think they call it. Kalakos: That's all but suicide. Cal: And if we're too far from each other and it's too late, vengeance has nothing to do with the Lord. Cain was wrong. I am my brother's keeper. And Niko's mine. Kalakos: That is suicide, plain and simple. Cal: You said it yourself, Father Kalakos. Niko is who he is because of me. I am who I am because of him. For me that won't stop. I'm terminal and Nik is my chemo. He keeps me human and he keeps me sane. Niko, I don't know what he would've been without me. Not you. Absolutely not you. Better off? Maybe. Able to love, like he loves Promise? To trust, like he trusts Goodfellow? I don't know. And neither does he. Kalakos: It's insane. Friends and family members grieve and go on when people die. They don't jump into the grave after them. People live on. Cal: Yeah, people do. Certain monsters and heroes don't.
The flashback in Doubletake that shows how a young Cal got medicine for Niko when he was sick. He had heard their neighbor was a nurse who got fired for stealing medicine, so he went over to her house and turned on the waterworks. She brushed him off, whereupon he realized she was 'a Sophia'. So he stopped crying, wiped his face off, and calmly told her of all the valuables Sophia had in her house, and added he knew how to call 911 and that 'police don't like people who hit little kids. Or touch them in bad places like it says on TV.' That gets her to cooperate, and she comes over later with some newly-stolen antibiotics. Cal takes her and shows her Sophia's valuables, and matter-of-factly tells her she should sell them before Sophia comes back and cuts her to pieces, because she's done it before, and he'll say they were robbed as a cover story. The ex-nurse is thoroughly creeped out by him and all but runs away from the house, but Cal doesn't care because Niko got medical attention and lives. Oh, and did we mention he did this when he was four?