Of all the villains Sam and Dean Winchester have faced, Lucifer remains the best baddie of the Supernatural bunch. Although the devil wasn't the first arch villain in Supernatural, the show's initial 5 seasons build gradually towards Lucifer's debut. From childhood, Sam was prepared as Satan's vessel, seasoned with demonic blood and groomed by Azrael, the yellow-eyed demon that killed the Winchesters' mother. Sam and Dean fought valiantly against the likes of Azrael, Lilith and Ruby to avoid Lucifer rising from the cage, but ultimately failed, and the evil archangel was formally introduced at the beginning of Supernatural season 5, taking the guise of Mark Pellegrino's Nick.
After acting as the main villain of Supernatural's fifth season, Lucifer was eventually locked inside The Cage once again, but his incarceration would prove a temporary arrangement, and the devil has resurfaced time and time again to gleefully bother the Winchester brothers. During his time, Lucifer has taken over Sam's body, plotted to end the world, briefly taken over Heaven, tried to corrupt his half-human son, and turned Crowley into a humiliated demon slave. Bigger villains have come and gone in Supernatural; Amara, God and Apocalypse World Michael have all proved worthy adversaries to the Winchesters, but as Supernatural nears its final chapter, Lucifer remains the best villain the series has ever produced.
One aspect that makes Lucifer so wickedly compelling are the various faces and personalities he wears throughout Supernatural. The Mark Pellegrino "Nick" variation is clearly the default setting, and that incarnation of Lucifer alone is incredibly effective. Pellegrino is able of exuding innocence and fun one moment, before turning on a dime and spitting venom and menace the next, enticing viewers with funny insults and a personable attitude, only to revert to his usual villainous ways. When he's not inhabiting Nick, viewers get to see other shades to Lucifer. Inside Sam Winchester, his ideal vessel, Satan takes on a more aloof and regal persona, while his stint as a rockstar highlighted more manipulative traits, even if this particular arc proved a little divisive among fans. Despite being part of Supernatural's world for 10 seasons, Lucifer has consistently been one of the most interesting characters by always surprising audiences, keeping both the Winchesters and viewers on their toes.
Of course, the key to any good villain is making them a viable threat, and this has been a problem in Supernatural previously. Back in the single-digit seasons, demons, hellhounds and angels posed a major problem for Sam and Dean, but are treated as minor annoyances in the current timeline. This is to be expected from a 15-season story, and the constant need to find bigger and badder villains probably contributed to God being installed as Supernatural's very final bad guy. As such, it's impressive that Lucifer has managed to buck this trend, and even when Supernatural has played up more to the character's comic chops, the writers always find a way to make the devil feel fearsome again.
Because Supernatural can explore so many different facets of this versatile and evergreen Lucifer, the villain moves in unexpected ways. Especially in Pellegrino's performance, the character is witty, self-depreciating and, very occasionally, relatable. Most of Supernatural's other main villains have towed a more conventional line - either the outright dominating power of Michael and Amara, the devious scheming of Azrael and Dick Roman, or the madcap other-worldliness of Metatron and God. Lucifer has many more strings to his bow, encompassing all of the above, and a more down-to-earth, carefree swagger all of his own.
Most telling of all, Lucifer is the only Supernatural villain who would be impossible to replace if removed. Any top-rank demon could've played Azrael's role, God only becomes integral towards the end, and the likes of Amara, Metatron and Lilith could be replaced by lookalikes with the same abilities easily enough. Take Lucifer out of the Supernatural fabric, however, and the show loses a huge piece of its puzzle, and likely wouldn't have enjoyed the same level of success. Perhaps if Crowley hadn't turned into more of an antihero over time, he might've claimed the accolade, but after the contract-loving crossroads demon died a hero, Lucifer is left as Supernatural's greatest foe.
Supernatural season 15 is currently on hiatus due to the current pandemic.