Crowley
Fergus Roderick MacLeod (c. 1661 - 1723) was a human, who after death became Crowley; a powerful demon who was formerly King of the Crossroads and later became King of Hell.
Although he initially served with Lilith, he went rogue in Season 5 after her death and Lucifer's release, aiding the Winchester brothers in resealing the latter back into his cage. With the power vacuum in Hell unoccupied, he appointed himself the new King of Hell.
Crowley's role is often shifting from main antagonist to anti-hero. He was an anti-hero in Season 5, one of the two main antagonists in Season 6, an anti-hero in Season 7, the main antagonist of Season 8 and an anti-hero of Season 9 and Season 10 once more. However in all of his durations as an anti-hero he is still openly an antagonist usually pulling a life threatening stunt on the Winchesters after assisting them. He is currently the longest lasting antagonist in the series.
Source: Wikia.