Is the North Star the brightest star in the night sky?

No. Polaris is not the brightest star in Earth's night sky. That distinction belongs to Sirius. Polaris is important because of where it appears, rather than exceptional apparent brightness.

NASA explains that Polaris lies close to the north celestial pole, near the direction in which Earth's rotational axis points. ESA identifies Sirius as the brightest nighttime star, while the Royal Observatory identifies Polaris as the brightest star within Ursa Minor specifically.

Being brightest in one constellation is different from being brightest across the night sky. Polaris helps observers in the Northern Hemisphere find north because it remains in roughly the same position as the surrounding stars appear to move.