1) Instantaneous destruction of the brain. Reptile brains are small, so crushing is probably the safest way to ensure that this is humane and fast. It's absolutely HORRIBLE...but this is about eliminating suffering, and not our feelings, isn't it?
2) Euthanasia by a vet--the snake will have to endure the stress of transport, and a needle stick, but then the anaesthetic should kick in. Don't let a vet inject your snake into the heart without using an anaesthetic first, that's just cruel.
3) Freezing, decapitation, CO2, etc--all cruel.
Decapitated snakes' heads have been known to react defensively to stimuli, which means the head isn't just still alive, it's fully conscious. Stabbing it in the brain AFTER you cut off its head is a bit too late, isn't it?
Not the most pleasant subject, but probably an important one.