Warhammer 40,000 – While the big flashy fancy units of the Tyranid horde do get much of the attention, the little swarms that make up the bulk of the army are nothing to scoff at. Throw enough bodies at anything and the enemy will run out of bullets eventually.
Gaunts of both varieties can swarm opponents making tanks ineffective trapping units in a combat and surrounding them so falling back is impossible, enough of them can even bring down a Primarch.
If you roll enough dice eventually you'll take down something. The Gaunts with flesh borers or devourers can put out a scary amount of shots, which will eventually cause even 2+ saves to fail.
Combine these little beasties with the bigger one that brings back models that died and you have a recycling nightmare that would make Captain Planet smile.