.22WMR
Quiet enough to use without ear protection outdoors, cheaper than 17HMR, .223 or .22-250 , jacketed bullet design instead of plated bullets or pure lead, enough power to take down critters ranging from squirrels to large fox, bobcat or coyote. With standart loads you get max range of 120-130 yards depending on ammo.
Bullet design is also superior compared to the .22LR, you get SP's, PSP's, FMJ's, V-MAX's, JHP's.
Compared to other low power varmint calibers the .22wmr offers you the biggest bullet selection.
With 30 grain V-MAX bullets you get frangible bullets @ 2200 fp/s that only penetrates 6 inches and explodes like a frag grenade, great for small varmint and it also very accurate. +
With 40grain softpoints @ 1900-2000 fp/s you get a bullet which penetrates up to 17'' and mushrooms up to ~cal .308
If you choose 40grain FMJ's you can shoot through 3.5'' pine wood!
Its also the most powerful rimfire round today with 355,20 ft/lbs