"Eye of the Storm", the final mission of the Protoss campaign in the original StarCraft. Raynor and his surviving Terran troops teaming up with Tassadar and his surviving Protoss to come together to try and defeat the Overmind. Controlling two races, each on opposite sides of the map made it really feel like a great moment. Not to mention the aftermath and Tassadar's crowning moment of awesome.
Raynor:Well, I guess all I have left is to see this through. The Zerg have taken everything from me. My home, my friends, my family. I know that nothing I can do can bring those things back but I'll be damned if I just sit on my hands and wait for the end. I want a piece of them all right. I'm in.
Don't forget this precious piece of dialogue from Brood Wars.
Fenix: It is difficult to believe that I am working along side my greatest enemy in order to save this sector.... Fortune has been whimsical of late.
Raynor: You sound like a tired old man, Fenix!
Fenix: Don't let the fact that I am 368 years older than you dull your impression of me young Raynor, I can still - how do you Terrans say it? - 'Throw down with the best of them!'
Raynor: I stand corrected.
Raynor talking back to the Protoss for mocking him.
Tassadar: Arrest me? Aiur burns at the touch of the Zerg, and you come all the way out here to arrest me?!