Lethal Injection
might not be that bad

Historical forms of execution include:
Buried alive
Crucifixion
Flayed alive
Scourged to death
Stung/bitten to death by insects
Slow disembowelment
Roasted on grill
Dragged from chariot
Torn apart by wild beasts
Rolled downhill inside spiked barrel
Death of a thousand cuts
Hanged by the neck until dead