“Each creature on the battlefield with the greatest power” may be a single creature if each other creature has less power.

The ability will trigger only if you control each creature on the battlefield with the greatest power as the beginning of combat step begins. If another player controls a creature with the greatest power or tied for the greatest power at that time, the ability won’t trigger at all.

The ability will check its condition again as it tries to resolve. If, at that time, another player controls a creature with the greatest power or tied for the greatest power, the ability won’t do anything.

The ability can target any creature an opponent controls, not just one that’s tapped.

Gaining control of a creature doesn’t cause you to gain control of any Auras or Equipment attached to it.