Sometimes hard to tell if he's anti-hero or just straight out villain.Gully Foyle.
A few other names that spring to mind for me...
Brandin from Guy Gavriel Kay's Tigana and Melisande from Jacqueline Carey's Kushiel's Legacy, for similar reasons.
Namely, they are extremely charismatic and intelligent. They're the sort of villain you kind of don't want to be a villain because of how admirable they are... but ultimately, there is a core of selfishness and ruthlessness that puts them right there. I enjoy the ambiguity, the way we almost question "they're not that bad really"
I put him in third place behind Ineluki from Menory, Sorrow and Thorn and Mordred from Arthuriana.In before @Toby Frost can say him - Steerpike from Titus Groan is well up there.
I have only read "Klara and the Sun" by this author. I haven't read "The Buried Giant" yet.in the novel The Buried Giant by Kazoo Ishiguro , Wistan the Saxon Warrior fits the antihero mold and, he's quite compelling .