With their father dead (traded for Dean’s life), the brothers become true independents. This season introduces the "Special Children"—psychics bred by the demon Azazel. It also gives us the first angelic weapon (The Colt) and the tragic figure of Ava Wilson.
Originally intended to be the series finale, Season 5 follows the brothers as they try to stop Lucifer and Michael from using them as "vessels" for a world-ending showdown. It’s widely considered the creative peak of the series. Season 6: The Aftermath Supernatural all seasons 1- 2- 3- 4- 5- 6- 7- 8- 9