CO2 is an easy and effective way, that when done properly is effective and completely irreversible. It is easy to set up, and literally doesn't require anything beyond patience waiting for the animal to pass. You don't even have to watch the animal while this is done.
Cervical dislocation is recommended for small animals, but can be done improperly resulting in an injured animal. It is not recommended for animals over 150 grams.
Smacking can go wrong too. If you have to smack a few times, it was not a clean death, and I can't see any reason to do it.
In my opinion, of those three methods, CO2 is easiest on both human and animals.
That said, I feed live. Snakes are perfectly capable and equipped with the catching and killing of their meal.
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