After a decade and a half, come to an end - and a conclusive one, at that. Over the final seasons, and especially the final episodes of season 15, the major players that fans had come to love (or love to hate) had their stories wrapped up. Some died, some sacrificed themselves, some were redeemed, some were lost - but all the books were closed.
However, not every story ended in a way that left fans satisfied. Some characters seemed to get a farewell that wasn't quite enough for their incredible importance to the show over the past decade and a half, while others got a proper (if heartbreaking) sendoff.
Fitting: Bobby
Bobby Singer has technically been dead since season 7, but that hasn't stopped him from being a main character. Since his death at the hands of Dick Roman, Bobby reappeared as a ghost, and then later as a Bobby from an alternate universe. In the end, though, Bobby's final scene on the show was meeting Dean in Heaven, where he wrapped things up in perfect Bobby style - giving Dean some guidance, some good advice, and some gruff love.
Deserved More: Adam
The third Winchester brother never really got a fair shake on the show - and his ending was no different. From the first episode (where he was revealed to have been dead from the start), to his abandonment in the pit, to his time as the vessel of the Archangel Michael, Adam was always the half-forgotten member of the Winchester clan. In the end, he had found a kind of peace coexisting with Micheal but was then snapped away with everyone else when Chuck decided to pull a Thanos on the Supernatural world. Chuck then destroyed Michael, leaving it a little unclear as to whether Jack was able to bring Adam back in some form, sans-angel, or if Adam was just gone.
Fitting: Jack
The Nephilim, Jack, was a recent addition to the show (only appearing in season 12), but he quickly became a beloved member of the Winchester clan. Despite this, he never truly fit in - he was a powerful weapon, a dangerous element, a creature of the like the universe had never seen... and in the end, he became the new God.
His ability to destroy Chuck by stripping him of all powers, restore the people to the world, and disappear into the ether as a new Almighty is about as close to a happy ending as anyone could have wanted for young Jack, and it worked perfectly.
Deserved More: Mary Winchester
In some ways, Mary got what she deserved - a happy space in Heaven, with John at her side, and some peace (finally). However, from a storytelling perspective, Mary didn't quite get the farewell fans might have hoped for. She was accidentally wiped out by Jack while trying to help him (and at a point where he could not control himself) - an event that was heartbreaking, and strained Dean's relationship with Jack to its limits. However, this was quickly (some would say, too quickly) resolved, and the resurrected matriarch of the Winchesters was written off the show for good.
Fitting: Rowena
Rowena may have started her time on the show as a straightforward villain, but by the end, she was a fan-favorite - and everyone enjoyed seeing her take her place as Queen of Hell (after an impressive redemption arc, too). In the end, Rowena taught Sam magic, helped save the world, sacrificed herself (proving that she had become a 'good' character), and then took over Hell with a great new wardrobe. This was definitely one of the happier endings for the series!
Deserved More: Charlie
The original Charlie got a heartbreaking end - brutally murdered while helping Sam and Dean. However, fans were thrilled when the character was semi-resurrected thanks to Apocalypse World, and came into Sam and Dean's world to carry on as a hunter.
In the final episodes, Charlie was disappeared by Chuck, and fans have to assume that Jack brought her back, but nothing more is really seen of her. Fans can assume that she and her girlfriend were both brought back, and lived happily ever after eating scrambled eggs and hunting things, but it would have been nice to see that.
Fitting: Castiel
Whether Castiel got a fitting end or not may be controversial - with some fans wishing that he and Dean actually had a chance together. However, overall, this is a truly epic end for an epic character. Castiel calls forth the Empty to claim him, saving Dean from Death, after a powerfully emotional goodbye where he confesses that he has always been in love with Dean. Destiel shippers rejoiced, but more than that, it was a remarkably human goodbye, a final one, and one that captured the essence of who Cas was in the end - more man than angel, and willing to sacrifice himself to save the ones he loves.
Deserved More: Dean
Dean Winchester spent his life as a pawn of God, a hunter with his father (and then brother, of course), a man who died repeatedly, was tortured in Hell, and who lost just about everything he ever thought he could have. In the end, he and Sam managed to take down God, but he only got a short period to actually live his own life (the series finale doesn't specify how long, but it couldn't have been more than a few months). In the end, he was killed on a pretty average hunt - and not even by a monster. During a fight, he was impaled on a metal spike, and quietly died in Sam's arms. In some ways, this was fitting - as he himself said, it was 'aways going to be this way'. However, it just seems unfair that someone who did so much for the world got so little for himself.
Fitting: Sam
If anyone deserved a happy ending, it was Sam - and he absolutely got one. The finale flashes forward through the rest of Sam's life after Dean dies, showing him having a son (presumably with Eileen, although her face is not shown clearly, so it could be someone else), a wholesome life filled with backyard catch, and then a peaceful death in old age. It's hinted that he still hunts - or at least, that he teaches his son (named Dean, of course) about hunting, because he is shown with an anti-possession tattoo as a young adult. However, he gets the simple, wholesome family life that he has always wanted. And after saving the world, he deserves it.
Deserved More: Crowley
In some ways, Crowley did have a great ending - a redemption arc that took him from enemy demon to King of Hell to frenemy of the Winchesters to a true ally. He sacrificed himself at the end of season 12, dying to trap Lucifer in another world, so as to save this one. It was an incredible moment, but it's incredibly disappointing that he wasn't brought back for a final goodbye. Throughout the final season, multiple characters from throughout the show's run returned for some kind of farewell, but Crowley was not among them. Seeing as Lucifer ended up able to escape the alternate dimension and wreak havoc, it ended up feeling a bit like Crowley's sacrifice was in vain, and there should have been some way for him to return for one last goodbye.