Cervical dislocation aka, a whack on the head ensuring that the neck hits a precise edge, such as the edge of a bench ensuring a good break. Quick, reliable (from memory bugger all practice was required) and easy.
Larger rodents, such as rats, require a more measured whack but a 100% first time kill rate can be easily achieved.
(Actually killing them quickly is easy... but with a bit of practice you can also do it so they do not bleed much so it is not messy.)