My vote is for Ignis, this thing is incredible, at least up to the 3rd sortie stage. I haven't found many situations where it's not the most effective way of killing shit.
In general most of weps in WF remind me of the old joke - how do you kill a mosquito? Well you can catch it in the air, garb its wings and put a tiny drop of glue on its mouth so it slowly suffocates. Or, as an alternative, you can also just swat it.
That's my experience with shit like Grinlok, Marelok, the snipers and all the powerful single-shot rifles and pistols. You carefully aim, slowly position your reticle on that tiny, jerky, erratically moving head and BLAM, first shot misses, BLAM second shot in the body, BLAM third shot hits the head. Hurray, I've killed a baddie in just 5 seconds, I'm the gratest.
And meanwhile some dude with an Ignis melts a platoon in the same time. Same for the spinning whackos with Maiming Strike Orthos or Hysteria Valkyres with Galatine, zooming all over the map, moving down dozens while you fiddle with headshots. Like, what's even the point of using anything else.