Usually you draw a line from the trailing edge (which is sharp so unambiguous) to the point on the leading edge that makes the line the longest.
The definition that makes the most sense, though, is to disregard the geometry of the wing and define zero angle of attack as the zero-lift angle, because then lift is proportional to AoA.
The definition that makes the most sense, though, is to disregard the geometry of the wing and define zero angle of attack as the zero-lift angle, because then lift is proportional to AoA.