Kel-Tec KSG – ★★★★★ Shotgun
If you want to stack armor, she's one of the best for it, with one of the highest base armor among shotguns. That combined with her skill, she can easily hit some insane armor thresholds, making her take 1 damage (zero with flash rounds) where other shotguns can struggle. This does come at a price of having her HP be on the lower end of shotguns.
However, there are now plenty of other options at this point in the game that can produce similar, but better results. That doesn't make her trash, but yeah. Still usable.
With all SG skills active, KSG does not actually have the highest armor, as her skill is the "Armor + EVA" version instead of M590's pure Armor version.
However, it's only a 10% armor loss and is active only 6 seconds every 16. Not to mention she has better base armor, and even if not by much, the significance of the difference increases after it's amplified by other Armor up sources like Fairies or FP debuffs from HGs.
When the enemies get super dangerous, having the capability to tank 1265 hits is actually ridiculously good. Overpowered, even. Basically the reason why you see enemies with super high AP running left and right, but the current armor system is flawed anyways.