The Frying Pan from Fable. It was full of lols.
Also, the Broom of Death from Assassin's Creed 2. How you manage to cut someone's throat with that thing, I'll never know...
In terms of totally useless weapons... I always thought that the Eridian weapons in Borderlands sucked.
The Pistol in Red Faction Guerilla sucked. In fact, a lot of weapons in Red Faction Guerilla sucked. If you didn't have at least the Sledgehammer (Which is locked into your inventory), the Assault Rifle, and the Proximity Bombs, you were screwed. The last slot you could play around with, but you'll probably just end up putting the sniper rifle or the rocket launcher in there.
Lucifer in Devil May Cry 4 was a piece of crap.
The remote Batterang in Batman: Arkham Asylum was useless, apart from getting the "Catch!" achievement. The other batterangs work fine, and they have auto lock on!
Any pistol in any Call of Duty game, other than those weird auto pistols in the new one and maybe the Desert Eagle.