It's safe to say that Buffy the Vampire Slayer doesn't get along with most vampires. That is true for even her relationship with the vampire known as Spike who, despite their romantic past, isn't beyond the reach of a well-placed and devastatingly accurate insult.
William "Spike" Pratt is a relatively young vampire by most standards. Also known by the nickname William the Bloody, he was born an aristocratic Englishman who was raised by his mother and aspired to be a poet, despite his lack of talent for poetry. Spike was sired by the vampire Drusilla in the 1800s, and despite becoming a literal bloodthirsty monster who personally killed two past Slayers, he retained much of his human passion and emotions. This eventually led to meeting Buffy Summers, the modern Slayer, who he came to love. Though she never truly loved him back, Spike found himself changing from villain to ally and to eventual friend and trusted confidant. This new heroism of Spike culminated in him regaining his soul and sacrificing himself to close the Hellmouth portal.
Spike would be later resurrected after his death, and in Buffy the Last Vampire Slayer #3, by Casey Gilly and Joe Jaro, he is shown once again fighting alongside Buffy in a future where vampires have vastly increased in power. Even in this future - where the two return to a flirtatious allyship - the Slayer is still not above hitting Spike with a perfectly crafted burn, referring to him as "Brosferatu." Despite appearing in comics rather on TV, this is a major candidate for the Slayerverse's best quip. It's a genuinely hilarious insult, referencing the English pop band Bros, who famously had slicked-back, platinum hair and sharp cheekbones. Spike's look was modeled after singers like Sid Vicious and Billy Idol, giving him both the look and connection to Britain. Combine that with the well-placed vampire pun, and it's almost like Spike was designed specifically so that Buffy could deliver this line.
Buffy is famous for its irreverent and witty dialogue, and that remains true even as the franchise expands to new worlds and potential futures. Buffy and Spike's bond is one of the series' most popular and most problematic, but Last Vampire Slayer shows Spike as the final member of the Scooby Gang who is around to help Buffy save the world. The series ends with the suggestion of a relationship to come, in which they will help train Willow and Tara's daughter as the next Slayer. It's a 'happy ending' some fans will love and some will hate, but one which doubles-down on Spike's adoration for Buffy. As hilarious of an observation as "Brosferatu" is, it underlines the Slayer's enduring affection for a man she has every reason to hate.
Buffy Summers is the most successful Slayer in the history of humanity's war against vampires, demons, and all the dark things that go bump in the night. However, much of that success is related not just to Buffy the Vampire Slayer's skill and abilities, but also to the allies she has gathered around her, and at the end of it all her closest ally - and sometimes punching bag - is none other than Spike.