Superman is physically much stronger than Batman. Superman has superhuman strength, can fly, and is nearly invulnerable, while Batman is a regular human at peak physical condition.

However, Batman has often been portrayed as capable of defeating Superman through strategic planning, using Superman's weaknesses (like kryptonite and magic), and his superior tactical mind. Their most famous confrontations usually show that with enough preparation time, Batman can level the playing field.

So in terms of raw strength: Superman wins by far. But in terms of who might win in a fight: it depends on the circumstances, particularly whether Batman has time to prepare.
