I know the worst way to go would be Darkseid's electric torture. Pain by nerve induction. Limitless, an eternity of escalating panic and misery until your brain fails to provide anymore feedback and shuts off.
Actually, imho, physical damage is a lot worse than pain for a person. Something about being burned registers on an emotional level, how the skin reacts and mutates. ugghh. Being ignited by Human Torch is the worst. Or maybe, as for damage to the body, Gorilla Grodd could do some nasty stuff. Chimps do the most damage to their prey in the wild because they have no sharp claws or fangs to kill efficiently with. Chimps have the least efficient killing method, and don't know what they are doing. They grab you, and they tear you limb from limb, and take little bites out of you while while you watch that. ughh. Death by Gorilla Grodd is the worst.
How would I rather go? Doctor Manhattan's molecular rearrangement, I won't feel a thing.