While The Winchesters has done the job of reintroducing fans into the world of Supernatural and continuing on the Winchester brothers' legacy. Although it has not been on long enough to craft a truly impactful villain for the fandom just yet, it does seem like the Akirda do have the potential to become just as memorable as the ones seen on Supernatural given all the creepy stories that have been revealed.
If the writers intend to make them just as great, then they may look end up spending time looking over episodes in the predecessor show as nearly every one of Supernatural's fifteen seasons gave fans a brilliant villain (whether they be the big bad or secondary antagonist). It'll be interesting to see if they manage to replicate these characters success or get inspired by them.
Season 1 & Season 2 - Azazel
Thanks to the monster-of-the-week episodes in the Kripke era, there are some terrific one-episode villains in the first two seasons of Supernatural and some great secondary villains, like Meg. However, none of them lived up to Azazel.
He is not prominent in terms of appearances in season 1. Yet, he is at the heart of everything, and his little screen time is fantastic. Then, in season 2, he proves himself as not just a powerful and intelligent villain but an intimidating, scary presence. He may well be the most important big bad in Supernatural since he kicks off the whole story.
Season 3 - Lilith
Given its shorter length due to the writer's strike, Supernatural's season 3 does not have as much of a 'big bad' influence as other seasons. However, Lilith still shines, particularly when taking up the classic creepy child form.
In terms of challenges to Lilith this season, the closest qualitatively is probably the Trickster - a testament to his brilliance since he only has one episode - or even the likes of Gordon Walker and the morally dubious Bela Talbot. With her role in the finale, impactful appearances like in "Jus' In Belo," and her general creepiness, Lilith stands out.
Season 4 - Alastair
Season 4 is one of the hardest seasons to judge regarding the best villain since Ruby proves to be quite an interesting character. However, she only reveals herself in the finale, so she is the best in hindsight, whereas Alastair is great as a clear antagonist during the whole season.
One of the most powerful demons in Supernatural and someone hugely important to Dean's story, Alastair had a levity and a frightening presence that captured the audience. Lilith is the 'big bad' of the season but does not work as well without the young girl guise, while Alastair shines alongside other secondary villains like Zachariah.
Season 5 - Lucifer
The whole of season 4 builds towards the arrival of one of Supernatural's biggest and best villains, Lucifer, who helped make season 5 of the show the most acclaimed of the series' run.
Lucifer pretty much has everything fans want from a villain this season. Not only could he be funny, ruthless, and unbelievably powerful, but he is also a layered individual with serious family issues and a wholly evil yet self-justified set of goals. Some Supernatural fans even find him sympathetic.
Season 6 - Castiel
Season 6 has a variety of villains to choose from, each giving something different, such as the humor of soulless Sam or the spoon-up-the-rear-end abhorrence of Raphael. Castiel and Crowley rule the season, but the former has a shock factor in his favor.
Crowley is perhaps, Supernatural's best villain and is his usual fantastic self in season 6. With his betrayal and God-like powers, though, Castiel is arguably the best of the season. Many believe the Seraph should not have turned bad, but it was something new and what really helped was understanding Cas' point of view - a trait of some of fiction's greatest villains.
Season 7 - Dick Roman
Despite their promising start, the Leviathan proved themselves the most underwhelming villains in all of Supernatural. Dick Roman, though, was a solid main antagonist for what may be Supernatural's worst season.
While he had the potential to be far greater and have more of an impact on the show, Dick inspired both fear and respect as the leader of the Leviathan, showing his ruthlessness and cruelty through the likes of 'bibbing.' A super monster if there ever was any on the show, Dick was a league above every other villain in season 7.
Season 8 - Crowley
Crowley is one of the best characters in Supernatural and is a consistently excellent presence in the show (even when he is not that season's big bad). He was just that in season 8, though, and thrived in the role.
Throughout the season, Crowley provides a slew of memorable (and often hilarious) lines while sprinkling in immense cruelty, influence, and evil while sitting on the throne of Hell. Crowley's arc is wonderful this as he gets so close to becoming human and, as a result, becomes a more layered and emotionally resonant character,
Season 9 - Metatron
The title of the best villain of season 9 is a tossup between the Scribe of God Metatron, or the physically terrifying Knight of Hell Abaddon. Fueled by the Angel Tablet and with more of an impact on Sam, Dean, and Cas' life, Metatron wins out.
He is the reason the angels fell to Earth, yet is smart and powerful enough to make himself look like the hero to many for much of it. While he can be a charismatic leader who believes in his own hype thanks to the Angel Tablet, Metatron is, overall, utterly hatable, sometimes annoyingly so. However, this only furthers his character - this season at least.
Season 10 - Rowena McLeod
The true villain of season 10 is the Mark of Cain, which Team Free Will fights valiantly to get off Dean's arm. The Mark of Cain could not bring the fire of Rowena, though, who debuts as a villain in this season before becoming a friend.
Rowena is not as physically dominant as the likes of Lucifer, but her smarts, sass, and skill in witchcraft make her a mighty opponent. The connection to Crowley and humor she brings opposite the Winchesters also help make her so great.
Season 11 - The Darkness
While its ending may not be as impactful as others, season 11 is a pretty great post-Kripke season of Supernatural, and the Darkness, a.k.a Amara, is the main villain of that season.
Her romantic dynamic with Dean was fascinating, if not weird, and the lore surrounding her and Chuck was great to watch. She may not have the pure evil of those like Lucifer, but is a powerful being unlike any other in Supernatural; ethereal and unkillable, and she is a big reason why season 11 was so good.
Season 12 - Lucifer
Lucifer ended up playing a key role in Supernatural long after season 5, being a big bad in multiple seasons, including 12 (in which he is the best villain by far).
This is not just down to the quality of Lucifer, though - especially since a lot of what he does this season misses - but rather because the British Men of Letters generally suck. Lucifer brings his usual mix of humor and evil incarnate to this season as he awaits the arrival of his son.
Season 13 & 14 - Alternate Michael
Alternate Michael is one of the strongest angels in Supernatural and a really great and unlikable villain who also allowed for Jensen Ackles to temporarily step away from Dean for a little while.
A ruthless militant leader in Apocolypse World and a classy but cruel being in regular Earth, alternate Michael turned out to be one of the toughest tests ever faced by the Winchesters. In terms of quality, alt-Michael surpassed regular Michael as a character with his turn as the big bad in these two seasons.
Season 15 - Chuck
The ultimate big bad of Supernatural and the villain of the controversial final season was God himself, Chuck - a whiny, petulant child who also happened to be the most powerful being in the galaxy.
Supernatural's take on God is an interesting one that really plays well in the final season despite its many flaws. He manages to be insufferable yet occasionally hilarious, all while providing enough threat to make audiences scared for the lives of Team Free Will. His defeat is one of the show's most satisfying moments.