The Mammoth tank's total confidence while being pitted against an alien invasion
Roll Over Them!
Lets have some FUN!
In a mix of Awesome and Nightmare Fuel, the Liquid Tiberium Explosion that happens after GDI uses the Ion Cannon on Temple Prime - the resultant blast launches crust debris into space, creates a Quarter-World-Wrecking Wave, and jettisons Tiberium Radiation into space in an ethereal glow. It's not hard to believe No One Could Survive That!.
And credit to Kane where it's due: That's exactly what he wanted GDI to believe, because that meant NOD wouldn't be seen as a threat as they stole all the alien tech they could get their hands on - and indeed, from that point onward in the GDI campaign, NOD is never the direct opponent with the Scrin being the focus.
The activation of SonicEmitters in the GDI campaign. Your powerful GDI force is on the run from the alien Scrin, and entire battalion of Mammoth Tanks are being destroyed in the background by the alien hordes while you are desperately fighting to evacuate a small group of scientists in buses with a small force of your own while the whole city is being torn apart. Your GDI troops flee across a river and an engineer reactivates the sonic grid meant to reclamate the area, not fight a war. The emitters power up, and you hear the trademark sound of TWIII sonic weapons firing... Cue Scrin units blowing up and essentially ending their ground pursuit of your forces, with soldiers shouting how powerful the emitters are in awe. That is one heck of an introduction to one heck of a base defense.
And it's not even intended as a base defense! It's meant to reclaim the Earth by shattering Tiberium through targetting its resonant frequency! The GDI engineers activate the grid just to buy you a distraction.. But the Scrin are partially made of Tiberium, so it turns into an utterCurb-Stomp Battle! And the best part? This Improvised Weapon is going to do exactly what it was designed to: reclaim the Earth.
GDI as a whole gets one. After several decades of peace, Nod comes out of nowhere, destroys their main command center (with most of their civilian leaders), takes the Ion Cannons offline and invades every single Blue Zone simultaneously. GDI takes the hit, fights off the assault, gears up, and pushes Nod back deep into the Yellow Zones. Then the Liquid Tiberium Bomb goes off and it looks like they've won... until the Scrin arrive, attacking cities all over the planet. GDI fights them off, turns around and successfully invades Red Zones destroying all of the Scrin towers but one. Kane may have succeeded in his objectives, and the Scrin may have been able to retreat, but on a tactical level, GDI crushed BOTH of them.
Even better if you get the good ending of the GDI campaign by refusing to use the Liquid Tiberium Bomb. Director Boyle essentially implores you to betray GDI's foundational principles to end the war quicker, thinking victory was impossible otherwise. Deciding to end the war the conventional way (or better yet, using the Ion Cannon) essentially proves that GDI doesn't need Tiberium Weapons, and leads to new and better leadership redoubling on their mission to stop the Tiberium threat.
The game is one long CMOA, but that's understandable when you've got an entire expansion focusing on that most Magnificent of Magnificient Bastards. When Kane reveals that he deliberately engineered Redmond Boyle's survival because he knew he'd act as an Unwitting Pawn and use the Ion Cannon on Temple Prime, Kane's capacity for the Xanatos Gambit reaches incredible proportions.
Alexa Kovacs briefly tops that by revealing that she set up Killian Qatar for an attack by Nod forces on Temple Prime, temporarily leaving Kane speechless.
"Our enemies believe that we have been defeated, and that I am no longer among the living. They are gravely mistaken. You and I are all that remain, but all that is necessary. For together we shall raise a great army. Together, we shall show the world's downtrodden who the real enemy is! GDI promises them prosperity, but in the slums of Rio, the homeless and the hungry know the truth. They have been forgotten. They have nothing!... but... their.. anger!... We will take that spark and start a fire which will become a great conflagration! PEACE! THROUGH! POWER!!!!"
All of the Epic Units can be so awesome to behold; to elaborate:
The GDI MARV (Mammoth Armored Reclamation Vehicle) is like a Mammoth. It has Tank Goodness, is much larger than a Construction Yard, can drive over Tiberium fields to harvest resources (especially useful for the opponent's field as it's not as efficient as dedicated harvesters), has room for four infantry units that each enhance the capabilities of the tank, and the MARV has a BFG that works on a similar principle to the GDI Sonic Emitter.
The NOD Redeemer has room for two infantry units which is fine but its main feature is its Rage Generator that sends enemies into disarray from turning against one another. The Redeemer also is equipped with a weapon similar to the Obelisk of Light that deals most single-target damage of the three Epics. For an Epic, it is somewhat quick too.
The Scrin Eradicator Hexapod is unique among the three in that it appears to be a living creature walking on six legs, and is armed with a damaging set of Zeus Disc Launchers, similar to the Devastator Warship. The Hexapod has room for three Scrin infantry units but is also able to "harvest" enemy units for a portion of their cost by killing them directly.