Decapitation
Though a very popular form of execution in history with the French and he Tudors using it extensively, it should be a pretty easy way to go. It’s not.
Sometimes the blade isn’t sharp, or the force isn’t enough to cleanly sever the spinal chord, and the person remains there, alive and partially decapitated. Countless stories have told the tales of guillotines and axes that have been unable to complete the job in 1 blow, leaving the executed there in excruciating pain, crying, screaming and some even still praying.
That’s not to mention the stories of disembodied heads that retain consciousness for some time after separation from their bodies. Some say a head remains alive for 30 seconds, some say only 3-5, but that’s still too long to be bodiless and in agonizing pain.