Simple and effective, reusable and quite painless if the condemned is first sedated on a gurney with common surgical anesthesia. Do not allow witnesses to view the beheading. Simply allow them to see the body from the shoulders down. Allow him to say his final words, knock him out with the drugs, then wheel him into the device face up so that his head goes through the opening. Begin the countdown and then have a doctor pronounce him dead once everyone sees his body shudder as the blade drops.
Public spectacles were a mistake with the original guillotine and undermine it's simplicity and effectiveness.