Weapon 0 would be whatever your gun switches to when you press the 0 key. As no weapon is switched to, even if you use a JJAmmo/JJGod/JJGuns cheat, I assume that there is no such weapon. On the other hand, maybe it's a super special weapon you can't get by cheating, like the blade gun.
As for the ultimate weapon, it's the laser shield, and I can post proof if you want.