What is the best opening for black?
"Caro khan" doesn't exist.
Except maybe in the Star Trek universe.
The best opening for black is to start first.
Or make two moves in a row.
Not making mistakes is more important than finding the best moves.
It depends on what White opens with and what fits your play style. For K pawn openings I like the simplicity of 4 Knights. For Q pawn I am experimenting with Kings Indian Defense as I find it gives me a lot of problems when I play Q Pawn as white.
What is the best opening for black?