My thought as well. I feel like if we had this conversation 10 years ago anyone who didn't mention the BFG right away would be labeled as insane. I mean people are listing guns that one shot people. The BFG would destroy every living being in the room and you didn't even have to aim. I dunno how any gun could possibly compare to that, at least in MP. In single player as someone mentioned we've had guns that destroyed entire planets, but as far as guns that really feel powerful the BFG just has to be at the top.
Also if we are only talking real world weapons then the AWP/Desert Eagle combo in CS would have to take the cake for me. I don't think I can think of a more rewarding gun to fire than the desert eagle.