User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-98.113.182.208-20120512155723/@comment-98.113.182.208-20120512162431

And also, Lucifer is the ONLY Major Supernatural Villian that the Winchesters couldn't kill, but had to seal up. God couldn't help and Michael wanted to start the apocalypse, so Lucifer was practically invincible. Lucifer survived an instant plot-device kill weapon, being the Colt. Lucifer, more than any other major Supernatural Villian, had the biggest build-up to him. Seasons 1 to 5 lead ulitmately up to Lucifer, Satan himself. Azazel, Lilith were both his creations and they were major villians of each of the earlier seasons.

Bottom line here - No major villian had a bigger arc story, no major villian was more unstoppable and unkillable. He was the only major villian who the winchesters couldn't kill.

The Leviathans don't live up to their hype and their arc is no where as big as Satan's himself.