The best measure of a star's brightness from the options provided is absolute magnitude.
Absolute magnitude measures a star's intrinsic brightness, which is the brightness it would have if it were located at a standard distance of 10 parsecs from Earth. This allows for a direct comparison of the true brightness of different stars, regardless of their distance from us.
Apparent magnitude measures how bright a star appears from Earth, which can be influenced by distance and interstellar material.
So, while both absolute and apparent magnitude are important, absolute magnitude is the best measure of a star's true brightness.