My least favorite weapon was the Bryar; with autoaim for other weapons it didn't have the increased accuracy to justify its weak attack. Using it was like bringing an ugly water pistol to a Matrix gun fight.
The Fusion Cutter comes in a close second due its weak attack. The only thing that saved it was that it looks damn good doing it.
The Mortar gun comes in third because it is bland and awkwardly large with a boring explosive attack. However, I found it quite handy for scattering lower level grouped enemies in close quarters. And I loved the sound of it firing.