The M9 from Ghost Recon. Bad range, bad damage, crappy aiming, and the AI would automatically switch to it if they had it, even if they had an SAW or MG3.
Not so much a WEAPON, but...
Mutalisks in StarCraft. They have crappy range, mediocre damage, light armor, and cost (theoretically) 5 times as much as a marine (100 minerals, 100 vespene, 50 for the extractor), and 5 marines with U-238 will shred 2-3 mutalisks.
Not so much a WEAPON, but...
Mutalisks in StarCraft. They have crappy range, mediocre damage, light armor, and cost (theoretically) 5 times as much as a marine (100 minerals, 100 vespene, 50 for the extractor), and 5 marines with U-238 will shred 2-3 mutalisks.