I suppose another very loosely defined notion of closeness is the degree of human habitability, in which sense Mars might be closest. Both Venus and Mercury are

I suppose another very loosely defined notion of closeness is the degree of human habitability, in which sense Mars might be closest. Both Venus and Mercury are incredibly hostile environments, although one could argue for floating cities on Venus and a thin habitable zone at the poles of Mercury. I understand of course that anywhere "not on Earth" is incredibly hostile to Human life, at least what we can see with present day technology. For truly habitable planets, we might have to consider other star systems and even then there's no guarantee we'll find one. I don't know to what degree is the abundance of oxygen as a loose element a sign of life, but I'd expect it to be bound to minerals anywhere without significant plant-like life. Perhaps finding another habitable planet is the same task as finding life on another planet?