I haven't played Halo is years, so my memory may be wrong, but I am going to say Felwinter's Lie. The only reason that the shotgun from Halo was overpowered was because it was the only shotgun, so you couldn't counter.
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And lots of things could counter the shotgun, pistols and snipers, rocket launchers, vehicles even. The flamethrower from the PC version could even postmortem kill a shotgun user if it didn't beat them outright.
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More to it than that. As far as I can recall it had both incredible range, stoping power, and rate of fire, coupled with a 12 round capacity, you could kill a room full of anything.