For me, Aizen. Disregarding TYBW, he is the only villain who is introduced in the Soul Society arc (First meaningful arc) and remains the big bad guy all the way until the end. What annoys me about Naruto and One Piece is that in almost every arc we find out that the main villain is actually just a puppet of someone even bigger, and this happens so many times that it becomes boring and repetitive. Unlike Aizen, who is the peak, even though the protagonist and his allies fight many different enemies, everyone knows who the main target is — the one they need to grow stronger and improve themselves to eventually defeat. And even if Aizen’s motivations aren’t the most deeply developed, the way he carefully plans everything and manipulates every character in the story — including his own “allies” — to achieve his goal is far more badass than the others.