I feel like it’s Kira, because he kept his secret for 15 years and when he did get caught, he changed his identity through Cinderella, but he only lost due to his ego (and fetish)
In terms of "Smartest" probably that goes to Kars considering he has an IQ of 400 but Dio is easily the smartest one intuitivly or in an intuitive situation. He literally mastered the heaven plan which literally is a very abstract concept that is extremely complex. I'm actually giving this to Dio actually, I mean Kars has an IQ of 400, but even with that it could just mean he's logically smarter rather than intuitively and really hasn't proven to achieve a feat like making them plan of a transcended abstract concept like Dio. Definitely Dio
