I tend to agree with Arthwollipot on this, but I'm not sure one even has to. The founding fathers were, as usual, rather parsimonious in their language, and left the interpretation of "cruel and unusual" to the discretion of those who would use those principles in the future. No doubt some punishments once common no longer are, and some once considered within bounds no longer are.
I'm sure flogging is still practiced in some places, and that it once was practiced here as well. I cannot think at the moment of crimes for which flogging would be appropriate, but I do find it possible to consider that it might be a preferable form of punishment if the alternatives were suitably inappropriate. I you commit an offense that the authorities consider should be punished with twenty lashes, that might seem attractive if the alternative is a year in a dungeon. It does not make flogging a good idea for a crime that deserves neither. And introducing the word "spanking" into this discussion is, I think, far off base if one is speaking of the punishment of adults for crimes.
Of course how we view a punishment is going to vary, and all sorts of arguments involving responsibility and free will keep cropping up. We have seen elsewhere the question of how one should regard some speculative chemical or neurological cure for violence, and in the mean time we have to try to figure out the boundary between what is necessary and appropriate and what is not. I think that changes, in part as our own society changes, and also as we learn more. At the theoretical level, there will always be the "Clockwork Orange" debate, though. Imprisonment is, at least in theory, a deterrent not only to those considering crimes, but those who might repeat them. Execution is a deterrent only to the former, and there seems to be considerable evidence that doesn't work well.
Of course, in some degree, any restraint is going to be painful in some degree. Being imprisoned for your crimes is not going to be a nice thing. it involves some deprivation of freedom, after all. There is likely to be some mental suffering going on there. We can hope that it develops into an improvement in character and a reluctance to do the crime again, but along with being the only necessary thing we know how to do, it's going to be hard. But it's not physical suffering, and it's not dying. And it's not the psychological suffering of knowing you're going to be killed.
To imagine it's the same except for degree seems, if not a bit disingenuous, not very carefully thought out.
And I repeat again and again, even if you have decided that execution is the best or only sentence for certain crimes, I think it unnecessarily cruel and inefficient. The moment of a death sentence is ultimate enough. That's as hard as it gets. Anything other than a quick and painless snuffing after that is gratuitous cruelty.