Spike was, without a doubt, the best villain on Buffy The Vampire Slayer, and it wasn't just because he was funny and relatable. Most fans of Buffy loved Spike, even though he could, at times, be a polarizing character. He started the show as a full-fledged villain, but at some point, his goals aligned with those of Buffy and her gang, and they ed forces. Spike was regarded a hero towards the end of the show, but despite that, his character peaked as a villain.
Spike was the best villain on Buffy The Vampire Slayer because he had many relatable qualities at the root of it, even though he played his part in the toxic relationship tropes on Buffy The Vampire Slayer. He doesn't aspire to bring the world to its end, as many Big Bads on the show do. He has hedonistic tendencies but doesn't aspire to be the villain in everyone's story like most villains. Spike actually appreciates the world and everything it has to offer to him. He isn't too happy when his former companion Angelus endangers all that. His dialogue and actions critique the supervillain trope of a Big Bad trying to destroy the world. He's there to stop it, even if that means becoming a reluctant ally to Buffy and her friends.
Spike Subverted The Big Bad Trope
After Spike's initial introduction to Buffy The Vampire Slayer in "School Hard", which was one of James Masters' favorite episodes, it became clear he was intended to be the Big Bad, or at least a villain on the show. Spike's entire spiel for most of the early seasons was to try and kill the Slayer any way he could. This makes sense, as they're mortal enemies, and she's endangering the same world he likes living in so much (the same one he's trying to save from Angelus).
So the viewers were surprised when Angel lost his soul and Angelus took over that role, leaving Spike working to stop him. His motives for becoming an ally and going against the Big Bads are relatable. And while Spike is on the verge of being the Big Bad, the roles soon become reversed, and he finds himself among the good guys. The show's creators used this as a way to flesh out his character, give him more backstory, and make him the best villain on the show. However, his debut on Buffy The Vampire Slayer set up Spike's hero turn later on the show.
Spike's Motives Are More Grounded Than Any Buffy Big Bad
Though Spike didn't have a soul when he began his journey on Buffy The Vampire Slayer, his nature was romantic, and his motives were often rooted in love. He turned into a vampire due to a rejection by the woman he loved, and while being a vampire, he developed a strong bond with Dru, before getting the ultimate redemption arc by getting a soul. It almost seems like love is Spike's way of rebelling against the world. It is what sets him apart from the other vampires, who are often perceived as pure evil. He is a feared vampire, but Spike's original nickname better explains his ending, as he is also William The Bloody - the romantic poet with a heart that beats for love.
After Spike's introduction on Buffy The Vampire Slayer, it was no surprise that the secondary villain was given an arc in which he became a hero and sacrificed himself for the greater good. Even when initially teaming up with Buffy against Angelus, he knew it would be dangerous, and could potentially get him killed. Still, Spike's romantic and rebellious nature drives him to fight for the things he wants or believes in, which is only amplified when he gets a soul.