I was running a character creation session last night, and noticed the Gas grenade for the first time. For those who also haven't noticed it, you can put any toxin into the grenade and weaponize it. So I looked over the toxins to see what would make a fun grenade.
Pepper Punch is basically super pepper spray. It's incredibly cheap, very powerful, and easy to come in contact with (being almost impossible to protect against). Once afflicted, it is almost impossible to resist against the Power 11 (each hit to get to resist reduces the Power) and victims are incredibly likely to drop to the ground in burning agony. You are likely to be incapacitated for at least 3 turns by the grenade, if not knocked right out because of the large amount of stun damage. Also, once afflicted, all wound modifiers count as double for the next 10 minutes. Even if you can resist the effects, you still take some stun damage and suffer huge penalties.
Worse, because Pepper Punch is so cheap (5 nuyen per dose), you can add additional doses, forcing additional resistance tests every turn and increasing the potency of the grenade.
For a scant 45 nuyen, you can end a large number of combatants. Rather than just stunning your opponent, you are causing so many negative effects that they are going to be out of combat inside a turn. Even the toughest troll may not be able to resist, and would be easily dispatched afterwards.
The only complication I've seen is how protective gear works. Since the toxin is Contact and Inhalation, I'm not sure how effective a respirator or gas mask would be. Is it an all or nothing defense (requiring a chemical seal), or would a respirator still help?
Regardless, this is a brutally efficient weapon for a pretty low price.
The runner up is the Kamikaze grenade, strictly for the shear amount of havoc that it can cause by dropping it at your team's feet.