What is the brightest star (relative magnitude) in M31?
The Variable stars that Hubble studied in M31 when he showed that it was a galaxy are among the most luminous. They include Var-A1, a luminous blue variable, and magnitude about 16.5. Var A-1 is one of the most luminous stars known.

Nasa has a catalog of stars in the m31 field, The brightest star in this catalog is an 11.4 magnitude star. But I think that database needs careful interpretation.