I definitely second the minigun from F3. Even the unique one sucks compared to a regular AK. A minigun should make you feel like Arnold in T2. It only becomes adequate when you have 1000+ rounds for it. And the bolts in Stalker aren't a weapon, they were there to detect anomalies. It was a nod to the book, called Roadside Picnic. The usefulness of the machinegun in Doom depended on how you played. Yes, it did the same damage as the pistol, but much faster, and without losing much accuracy. I used it most of the game, didn't stop until I found the plasma rifle. It's a pale imitation of the machinegun from Quake 4, which is essentially a precision rifle. And the plasma cutter and force gun from Dead Space are monsters when you upgrade them all the way.
But the most underwhelming gun ever is the blowtorch(fitting) in Dead Space. It royally sucks. It was much easier to just beat them to death.