BFG: Oh, several. Let’s start with “that created by any excuse to shove antimatter rifle-grenades in your sluggun (see: Abnormal Ammo) or battle carbine and cut loose”, shall we? There are also a variety of multibarrel miniguns, and yes, hypothetically even some that you can shove antimatter rifle-grenades into (see: More Dakka).
Specific examples would include the S-11i Mamabear, a souped-up sluggun which requires heavy bone reinforcement of most species in order to cope with the recoil, but can pull off one-shot kills on just about anything you care to name; the E40 Motherstorm, a very overpowered electrolaser that is unsafe at any setting, but very useful against mechanicals if the environment will let you use it; every single hunting weapon ever on Paltraeth, the kaeth homeworld, where the apex predators are oversized velociraptors with natural scale-mail plating; and the EI-12d Valkyrie target designator, which on its own is a tiny weak modulated laser, but which if there’s an orbital defense grid or an assault cruiser owning the local orbitals, can unleash more hell than everything else in this entry added together.
In the Fire Breathing Weapons category, the plasma-belching sets-fire-to-everything-around power-armor-mounted weapon of doom that is the KF-11 Dragonspume. Even if its primary use is taking down cyberswarms and nanoswarms through thermal overload.
In the vehicle-mounted weapons category, that minigun-class weapon which is fitted to a G7-BU Sunhawk (see: Cool Plane) and which necessitates – as does the main weapon of the aircraft to which it is a homage – some special care in using to avoid find yourself flying backwards.
And any number of the one-off custom designs from Eye-in-the-Flame Arms, whose weapons designers (drawn, substantially, from the Cyprium-ith-Gislith line) consider the existence of any practical purpose for the weapon distinctly secondary to generating more and more extreme levels of overkill.