Big Boss surviving sustained marathons of electric cattle prod shock therapy at the hands of an inept civilian torturer, who is more likely to fry him to death out of jealousy and spite than extract the confession she wants out of him.
One for the player for surviving all three sessions and arguably breaking Strangelove herself.
While there are plenty of awesome moments in Peace Walker, one of the most memorable is the entirety of the 'Get to the Control Tower' mission, particularly when played in single player. The climax of the mission is when the player encounters their final obstacle; a squad of commandos and a chopper, all of whose sights are centered directly on you. Making it even more intense is that the player is likely running quite short on ammo, and the building the player enters through is promptly blown apart by the chopper, taking away most of the player's cover. If that's not enough, the main vocal theme of the game, Heavens Divide, begins playing in the background. It is probably one of the most intense and awesome moments of the game, possibly even of the entire Metal Gear series, especially when the player finally takes down the chopper.
The real kicker is going outside at the start, even though Miller says it's suicide, and getting on the mounted machine gun. YOU CAN'T GET HIT USING IT. Time to go Rambo.
And to make it even better, the mission ends with things getting tight for Snake, up 'til when the Sandinistas and MSF arrive, gunning down all the enemies in one of the best Big Damn Heroes moments in the whole series. Cue Amanda finally being recognized as "Comandante", and after she hugs Big Boss, everyone starts cheering for him. It's such a moment of glory.
This unforgettable moment happens just a leetle bit before the hug, it's too awesome for words so let it speak for itself.
Vladamir: You dare point your gun at a comrade?!
Amanda: We will not be pawns of the KGB. We will win our own victory!
Chico: ¡Hasta la Victoria Siempre!
Guerrilla: Amanda! We're home! Look, we're back in Nica! We did it Amand - I mean, Comandante!
Soldier: Boss, you hurt...?
Big Boss: I'm fine.
Amanda:(rushes over to him and utters an exclamation of joy)Vic Boss!
All of MSF and the FSLN:(at first whispered, then rising to a frenzy of a crescendo) "Vic Boss... Vic Boss. Vic Boss! VIC BOSS! VIC BOSS! VIC BOSS! VIC BOSS! VIC BOSS! VIC BOSS! VIC BOSS! VIC BOSS!
Taking down Metal Gears is always awesome, but taking down Metal Gear Zeke to Koi no Yokushiryoku, instead of the typical electronic music was pretty damned sweet.
There are QTE prompts when fighting Cocoon, Peace Walker and Zeke which result in Snake bench pressing the mech when it attempts to stop him and throwing it off balance. Understanably, the AI Pods and Paz freak out.
In hindsight, Big Boss' DNA does seem quite valuable considering he can stop five hundred tonnes of mech from reducing him to paste.
Kazuhira: How can anyone lift a monster like that?
Paz: It is a miracle!
The QTE at the end of Infiltrate the Underground Base. The prompts are so fast it's borderline impossible to avoid losing (and continuing to the next mission), but Big Boss' CQC Foe-Tossing Charge of eight assault-rifle wielding Peace Sentinel goons. (not to mention stealing Strangelove's ID card to escape later upon completion) is totally worth it.
Meta: Metal Gear Solid Peace Walker is one of the few games that got 10/10 from Japanese gaming magazine Famitsu. And it's a PSP game.
Whenever Big Boss gives an awesome speech. Especially the early one which is backed by the strings of the MGS3 main theme. "We gave up our homes. But we're still alive. We're still fighting. And there's always another reason to keep on living."
Big Boss: Amanda, join my unit until you've healed up.
Amanda: Your... unit? Who are you? Why are you here?