Hostile Waters/YMMV

Complete Monster: The Cabal, and the world leaders before 2012 in general. They created fake aliens as a form of control by terror, and during the protests of 2012 they had troops fire EM weapons at rioters, inciting them to violence, as an excuse to kill them. The cutscene where they unanimously agree they'd wipe out human civilization if they can't control it pretty much solidifies them as this.
Game Breaker: Puma + Cloak + Flamer = You win. Always. Simply drive up to the oil rigs, decloak for a quick burn, then recloak and run like hell. The enemy can no longer fight back except for defense. It works in almost every mission except the last one (barring the ones before the flamer). Even when the enemy respawns them, you just need to fly a plane over as a spotter, then rinse and repeat.
High Octane Nightmare Fuel: The disassembler, nanomachines reprogramed to rip a target apart from the atomic level. Your normal Grey Goo scenario just makes more nanomachines. This one leaves behind a blood-red puddle of goo and a quite literally melted landscape. And then there's the sound. Just watch.
That One Level: In the level where you return to island zero, If your first wave fails to cause crippling damage to the bugs' main production facilities, it becomes almost impossible for you to collect enough energy to clean the island. But the bugs, of course, have no similar problems.
- Well, not unless you abuse the fact they don't know you're there to knock out their oil rigs with a cloaked, flamer-equipped Puma.
- Also the isthmus mission. Recycle everything. Create a wave straddling the isthmus. Inch forward. Create carnage, a little bit at a time. Advance to protect the recycling units. Use almost all the recycled bits to replace thrashed units. Repeat oh, maybe a dozen times. Then hope there's enough energy left for your main assault.