I really really like Sauron and his own motivation and descent into madness. The fact he’s never wholly evil, capable of redemption and regret but his own fear stops him from achieving it is great. Then the madness that consumes him to where he thinks himself the new Melkor in the 3rd age is so interesting to me. Even at the height of his madness he only ever wants to control the world though, unlike Melkor who would render everything into oblivion.
He feels so human in comparison to antagonists. Just very relatable for someone we barely see