I think the reason people are (were) saying the shotgun from Doom (certainly why
I think it's the best) is because it is arguable one of the most versatile weapons from any game. A lot of games will have cool gimmicky weapons (like Turok's Cerebral Bore, for example) that win fans over simply for the coolness factor, but in the end you always wind up using the gun with versatility and availabilty. Doom's shotgun had tons of ammo lying around for it, could kill most enemies in a few hits, and was actually both a long and short range weapon. It might take some ammo, but the shotgun was actually the closest thing to a sniper rifle in the game. Defies logic? Yes. Great weapon? Also yes.
I apologize if the preceding was a "Captain Obvious" thing to say.
