The thumper is pretty interesting

[A_Clever_Monocle]

as far as I can tell its the only grenade launcher without a strength requirement, and really strong for when you get it. early game it makes killing a certain someone in depot A a breeze

[Ornias]

Considering you can upgrade it immediately after Depot A is just a bonus. It carried from me from lvl 8 to lvl 20 by itself on normal.

[Kiruha]

If you have high crit chance  and critical power feat, this weapon will be so strong so it will obliterate most enemis even on Dominating in 1-2 shots, better use it with special belt and Thermobaric grenades (for better range). Also you can use TM + adrenalin to shoot it 2 times per turn (25 for shot then 7 ap with belt to reload - that means each shot will cost 32 ap)

[Kajmak]

The reload cost is rounded up so it costs 8 AP, you could potentially do 3 shots with 2 reloads in between if you have a shell loaded beforehand, save 1 ap from the previous turn or use vitality powder opposed to adrenaline
3x25 + 2x8 = 91 AP