I have to say the Magnum pistol from Halo 3. In Halo 1 it was THE weapon of choice, good clip, good fire rate, good damage, good range with a zoom. In halo 2 it got worse. But in halo 3. It got really really slow in fire rate, the damage was awful and they even removed the zoom on it. Usually some of the weaker weapons like the SMG or plasma rifles were usable in dual wield situations, but there really wasn't any reason to pick up the magnum with another gun. Not even the plasma pistol as the chance of hitting that headshot right after using the plasma pistol wasn't reliable enough.