Best opening for black
There is no best. Merely a FINITE number of good options.
Against 1.e4 - Double King Pawn, French, Sicilian, Caro-Kann
Against 1.d4 - QGA, QGD, Slav, Semi-Slav, Nimzo-Indian, King's Indian, Grunfeld, Classical and Stonewall Dutch
Everything else is inferior at best for Black, and some are just outright bad!