Let's go oldschool: In the second to last orc mission of Warcraft II: Tides of Darkness, you are finally given your last missing piece of the techtree, and the Large Ham narrator announces in the mission briefing:
Fortunately... Orgrim Doomhammer has saved his greatest weapon... ready to unleash it upon the unsuspecting Alliance... at just this moment...
After that, the final mission's objectives are simply "DESTROY EVERYTHING YOU BEHOLD IN THE NAME OF THE HORDE!" Then the completely kick-ass Orc Ending Cutscene.
The Alliance campaign is not without its awesome moments, the most notable of which has to be the campaign's final cinematic of Warcraft II: Tides of Darkness, which depicts Khadgar unleashing his full power to destroy the Dark Portal.
The cinematic cutscenes that end each campaign in Warcraft III tend to be of the Crowning Moment of Awesome kind.
Arthas returning home as a war hero after a successful campaign against The Undead only to to walk up to him and stabbing him through the neck.
Arthas: You no longer have to sacrifice for your people. You no longer have to bear the weight of your crown. I've taken care of everything.
Terenas: What is this? What are you doing, my son?
Arthas: Succeeding you, father...
Arthas has another Crowning Moment of Awesome in the ending cinematic of the Undead campaign of Frozen Throne: Arthas shatters the Frozen Throne with Frostmourne and the screen goes black. It the shows the Lich King's Helm of Domination fall and slide to the feet of Arthas, who, picks it up and puts it on, after which his eyes shine with blinding blue light.
Nerzhul: Now we are one!
The first thing Archimonde does after arriving on Azeroth is destroying the wizard city of Dalaran just by drawing in the sand.
Grom Hellscream killing Mannoroth after Mannoroth taunted him in Warcraft III, freeing Orcs from their demonic enslavement. Mannaroth being so powerful that his body releases a huge burst of hellfire killing Hellscream is also awesome.
Hellscream: The blood haze has lifted... The demon's fire has burnt out in my viens... I have freed myself
Thrall: No old friend... You've freed us all.
Finally, the ending cinematic of Reign of Chaos, a Crowning Moment for Malfurion Stormrage, when he calls upon an army of thousands of Wisps to destroy Archimonde, using a World Tree empowered version of Detonate. Of course, Wisps don't work that way in the actual game.
One for Thrall is at the end of Warcraft III, where he confronts Archimonde, telling him that all that mattered was that the Orcs were free before being teleported away by Jaina, but not before hitting him with a lightning bolt. Considering that this actually hurts Archimonde, a commander of the demonic Burning Legion makes it pretty awesome in this troper's eyes. Even Archimonde himself is impressed by the puny mortal daring to stand defiant before a border-line Eldritch Abomination.
Khadgar, as an inexperienced if gifted apprentice mage, faces a bunch of orcs on his own in the swamp and manages to stand up to an orc warlock, who was likely far more experienced than him, until Medivh arrives and sets them all on fire.
Aegwynn's entire rampage in Northrend, shown in a vision of the past, where she tears apart the demons hunting the dragons, before she faces and defeats an avatar of Sargeras. Even if the last part was a part of his plan, it was still awesome.
As Lothar, Khadgar and Garona make their way down into the dungeons beneath Karazhan they're confronted by huge demon hounds, one of which makes the first move by trying to pounce on Khadgar. Lothar immediately steps in the way, draws his massiveruned sword and cuts the beast in half up the middle in a single upwards swing.
The final confrontation with the possessed Medivh deep beneath Karazhan, where after having been forcibly aged into an old man and having witnessed Garona being Mind Raped and Lothar set on fire, Khadgar manages to stab Medivh in the chest with a sword when he's distracted. However, as Medivh dies, Sargeras takes full control of his body and is about to burn Khadgar to death; cue Lothar rising up and cutting offhis head.
Lord of the Clans
When Thrall walks up to the man who taught him how to fight (after besieging his castle), grabs his earring (in response to the challenge he gives his students when they first meet to rip it off), then simply lets go of it and walks away.
Tides of Darkness
Turalyon gets a big one in the novel adaptation of Tides of Darkness. After Orgrim Doomhammer kills Lothar, Turalyon's religious doubts finally click into place when he realizes that the orcs are not of his world. This lets him finally make full use of his Light-given powers and blinds all the nearby orcs. After that, he picks up Lothar's broken sword, beats Orgrim Doomhammer into the dust, before taking leadership of the Alliance army and finishing the battle.
Lothar and Doomhammer's one-on-one duel during the Battle of Blackrock Spire is a drawn-out brutal melee which pushes both combatants to their limits. It ends with Doomhammer breaking Lothar's sword and killing him, but not before Lothar uses said broken sword to deliver one last blow that almost kills the Warchief.
This particular fight was written by a guy who also works for Black Library, so of course it was awesome.
War of the Ancients
In the War Of The Ancients trilogy, Jarod Shadowsong, reluctant hero and leader of the mortal forces, willingly stands against Archimonde when challenged by him, with full knowledge that Archimonde could crush him like a bug.