Here I have compiled a list of how much ammo it takes to kill an armored stalker.
Credit to Raga_BuildCraftUrbanist and Silberklinge for helping me out with the weapons I didn't have.
Primary
Simple shotgun - 11 shots
Best friend - 43 shots
Ak-48 - 34 shots
Crystal Mega Destroyer - 7 shots
Rambo's rifle - 33 shots
State Defender - 40 shots
Swat rifle - 43 shots
Marksman M2 - 25 shots
Deadly candy - 4 shots
Auto Peacemaker M2 - 47 shots
Plasma rifle - 17 shots
Eindringling - 28 shots
Verdict: The Peacemaker M2 and the Marksman M2 are the way to go.
Secondary
Pixel gun - 25 shots
Old Revolver - 14 shots
Deadly Viper - 13 shots
Crystal fast death - 40 shots
Amanita - 17 shots
Old comrade - 13 shots
Alien gun - 16 shots
Powerful gift - 11 shots
Sparkly Blaster - 10 shots
Flower Power - 10 shots
Plasma Pistol - 10 shots
Verdict: The Powerful gift seems like it's the best, but due to its small clip size and slow walking speed, the Amanita and the Old Comrade. The Sparkly Blaster and the Plasma Pistol are also good, due to their high firerate and their damage radius.
Special
Magic bow - 9 shots
Crystal Crossbow - 6 shots
Heavy machinegun - 53 shots
Guerilla rifle - 6 shots
Ultimate headhunter - 3 shots
Angry Reptile - 3 shots
Doomsday flamethrower - 34 fuel
Chain thunderbolt - 32 battery charge
Ptrototype PSR-1 - 2 shots
Verdict: The PSR is not the best gun. It's good but the chain thunderbolt is better. Why? Because of the insane amount of points you get. It does damage so fast that you can get 30000 points if you're lucky. That's 30 coins.
Premium
Dark force saber - 8 hits
Archimage Dragon Wand - 7 shots
Anti Gravity blaster - 5 shots
Cherry bomb - 4 shots
Big Buddy - 2 shots
Aramageddon - 2 shots
Firestorm G2 - 3 shots
Bass Cannon - 2 shots
Golden Lasor Cannon - 45 shots
Verdict: Armageddon or the Anti gravity blaster. Why the blaster? Because of its huge explosion radius, it can kill the birds too.