I find it interesting that the two currently most used methods are, to me, actually the most painful to the criminal. Not necessarily from a physical aspect, but psychologically. They not only have to go through the whole "pomp and circumstance" of the slow walk, being strapped in, etc., but then when the word is given, they're still conscious and aware for a good while as the gas is pumped in, or the cocktail injected.
If the people who complain about the perceived suffering that the death penalty causes really cared, they'd be more in favor of the older, "messier" methods than they would of current methods, because while "the walk" is still there, once the word is given, it's lights out. With the possible exception of a firing squad.
I think if I were on death row, I would take the nail gun they use on cattle over a firing squad. However, anesthesia plus nitrogen would be my preferred choice if I was looking for an easy death. Because with nitrogen alone, while physically painless, I'd still be anxious over the time it took for the gas to do its thing. Not that I'm necessarily against someone who has committed a heinous crime thinking about that during their last minutes.