Pretty much every scene where John is fighting counts, given the sheer number of people who are trying to kill him, as well as the cold precision with which he dispatches his foes. Baba Yaga indeed.
The Red Circle Club shootout is a standout, with John ripping through several dozen goons without even slowing down (at least until he gets to Kirill). Of particular note is when John attacks a mook, only to find that his gun is empty. Undeterred, he jabs the mook in the face with his pistol, stunning him long enough for John to reload and shoot him in the face.
Marcus gets one towards the end of the movie. After being tortured by Viggo for refusing to kill John, he still manages to get up and take out a few mooks before he's finally killed; even Viggo is impressed. And even then, it's suggested he probably would have gotten out alive if it weren't for the presence of Ms. Perkins.
Aurelio punching Iosef when he finds out that Iosef stole John Wick's car. He then stares down a gun pointed at his head by Iosef's chav sidekick and even pulls the barrel right up against his own forehead, demanding that the man either shoot him or get out of his shop.
Look at the expression on Iosef's thug's face while he's pointing the gun at Aurelio. He's the one with the gun, yet the thug is the one who's afraid.
Viggo calls him later, tersely asking whether Aurelio had struck his son.
Aurelio: Yes sir, I did. Viggo: And may I ask why? Aurelio: Yeah, well - 'cause he stole John Wick's car, sir, and... killed his dog. Viggo: (Five second Beat) Oh.
It's a subtle detail, but when John walks into Aurelio's, every mechanic in the garage stops what they're doing and watches him. They know what's about to happen and how badly Iosef fucked up.
This scene is also one for John, because you can see on Aurelio's face that he knows that whether he takes the car or not, he's fucked either way. But if he has to choose, he chooses to make an enemy of a mob boss rather than John Wick.
Every fight between John and Kirill. Three encounters, and each time they're pretty evenly matched. It's interesting since Kirill first appears as a random mook before becoming something between Elite Mook and The Dragon.
Before shit goes down at the Red Circle, Kirill tears Iosef a new one for being appallingly cavalier about the mess he's created. He looks like he's about to shoot Iosef himself right there, and the viewer is likely nodding along to everything he says.
Kirill: My job is to protect a boy, not babysit a drunkard.
Iosef: Are you scared of the fucking Boogeyman? I'm not.
Kirill: No? But you should be.
The Continental getting revenge on Ms. Perkins for breaking the rules.
Winston: Your membership to the Continental has been, by thine own hand, revoked.
[The four man firing squad executes Miss Perkins from four sides at once. Without missing a beat in his stride, Winston hands a coin to Charlie and his cleanup crew as they enter to dispose of the body.]
The entire criminal underworld system of rules count as a CMOA.
Viggo: *mockingly* Baba yaga. *gets up* It was just a fuckin' car, just a fuckin' dog.
John: Just a dog...? Viggo.
John: When Helen died, I lost everything. Until that dog arrived on my doorstep... A final gift from my wife... In that moment, I received some semblance of hope... an opportunity to grieve unalone... And your son... took that from me. Stole that from me. Killed that from me! People keep asking if I'm back and I haven't really had an answer. But now, yeah, I'M THINKIN' I'M BACK. So you can either hand over your son, or YOU CAN DIE SCREAMING ALONGSIDE HIM!
Viggo mentions that John was so good at what he did, that he once killed three people with a pencil.
Topped by what Viggo describes as how John worked his way out of the mob. When John wanted to quit, Viggo gave him one last assignment that was deemed nearly impossible. Nearly. Whatever job John did to be with his wife, it left behind a mountain of bodies and cleared the way for Viggo's criminal empire to become as powerful as it was. There's a reason why Viggo is utterly terrified of John.
Iosef getting his. For all his tough-guy posturing in the beginning, there's no final shootout or epic fistfight when John finally catches up to him. He gets the kind of coward's death he deserves: one bullet in the gut, and then one in the head while whining that "It was just a fuckin'—"BLAM!
The best part is that John wastes no words on this little prick. He just walks up to him after gut-shooting him, executes him like the punk he is, and then just walks away without even a second glance. No Pre-Mortem One-Liner, no Bond One-Liner, nothing but stone-cold silence.
John's Ironic Echo when answering the phone of Iosef's friend that John just killed, which terrifies the little prick even further:
Gentlemanly Charlie with his fedora and his cleaning crew quickly and efficiently cleaning up John's house and wrapping up all the bodies for disposal in what seems like no time at all.
In a meta-example, Keanu Reeves's acting! For almost always being disparaged as an actor limited to Dull Surprise and lack of talent, he gives a great performance in John Wick, almost universally received and happily so.
In addition, his ability to do a lot of his own stunts in both films while in his fifties.
Take a look at the poster for the second movie◊. Does John Wick look the least bit worried at all the guns pointed at his head? Does that look like enough guns?!
Taken even further with the third movie's poster◊. Except by now audiences are firmly thinking, "Seriously. Not enough guns."