The final mission in MechWarrior 4: Vengeance. You've just fought your way through an entire world in your crusade to free your planet from Steiner influence, crossing frozen tundras, deserts and swamps before moving in on the capital city. Finally, you storm the palace itself, all the while hearing the pathetic Smug SnakeBig Bad's increasingly desperate pleas for help and useless attempts to save himself. When he finally tries to flee in a DropShip, Ian Dresari delivers the most awesome response to a We Can Rule Together offer ever.
Roland: Mr. Dresari, stop this attack! If you desist, you can have William's place on the throne.
Ian: Sorry, Roland. Pass.
Roland: M-Mr. Dresari, spare me, and I promise you can rule without my intervention!
Ian: You still don't get it. This isn't about the throne, Roland.
Roland: Mr. Dresari, stop! If you don't want to rule...What do you want?
Ian: My family back.
The Mercenaries expansion for MechWarrior 4 includes many awesome moments. One of the very first missions involves escorting a convoy of freshly minted artillery tanks across rough terrain to a nearby base for delivery. Once there the base quickly comes under heavy attack from multiple lances of Medium and Light Mechs, too many for the paltry defense grid to handle. Then, the Hrothgar DropShip in the middle of the base powers up, opening fire on the incoming enemy Mechs with Gauss Rifles, PPC's and HMRM's. Needless to say, the ensuing carnage is beautiful.
Any time you take on rival mercenary group 'The Black Cobras' in Mercenaries, starting with a small scale raid against a facility defended by them right up to facing down Black Cobra leader Colonel Burr himself on a desolate moonbase.
Another Mercenaries example. One mission line involves retaking a planet from the Jade Falcon Clan. Eventually Spectre will challenge the Falcon battle leader to a Trial of Possession in order to resolve the dispute.
The other, much more awesome option is to do the challenge properly. Eight of your own Mechs, more than likely all Ace Customs by this point against ten fully outfitted Heavy and Assault type Clan Mechs, with fire and missiles blasting everywhere. Definitely one of the best levels ever.
And if you manage to pull it off, you can claim the enemy commander as "Bondsman", making her basically an elite pilot in your roster that you don't have to pay.
The voice actor for the Solaris Arena in MechWarrior 4: Mercenaries, "Duncan Fisher" will be voicing the announcer for the MechWarrior Living Legends Solaris Arena deathmatch mode!
The entire campaign of MechWarrior 3 is pretty much this. Starting with a botched drop that leaves most of the force scattered and/or dead, your player character must fight his way out with a measly Bushwacker dropped alone in hostile territory. You get supplies from the enemy bases you raid, your lancemates are survivors, just like you. In the end, even though your mobile field base (three repair vehicles with some outdated intel) is all support you get, and even though you get outright left to die by the rest of the strike force, your lance of four manages to kill or otherwise cripple every important enemy and installation AND commandeer a command shuttle to get out of there. Awesome.
One of the endings to Mechwarrior 2 depicts the climactic battle of Tukayyid between Comstar and the Clans. The cinematic opens with a Kodiak and a Timber Wolf retreating through a canyon, pursued by a Com Guard force including an Atlas, Mauler and a Catapult. The Timber Wolf stops, turns around and unleashes a devastating missile barrage against the Atlas...which simply walks straight through them without flinching and blasts the Mad Dog and the Kodiak to pieces.