My weapon I choose is Gilgamesh's noble phantasm from Fate/Zero and Fate/Stay Night. It's not just one weapon, it's a horde of weapons that can shoot at my enemy, or enemies, simultaneously. Pretty awesome, if I do say so myself.
But if I had to just stick with one, than I would choose Archer's phantasm from Fate/Stay Night [UBW]. It's pretty awesome as well.