Question

which is more fundamental mass of a body or its weight? why?

Answer 1

Mass is fundamental. As it remains constant in different physical conditions.
Gravity on the other hand varies according to the acceleration due to gravity.

Answer 2

Mass of the body is the fundamental quantity. It value remains same at all places.

Whereas, weight of a body depends upon the mass and value of acceleration due to gravity at the place where weight is measured, i.e., W = mg. As we know that value of acceleration due to gravity is different at different places. It decreases with height, its value is  more at poles instead of its value at the equator. Hence weight will vary according to the value of acceleration due to gravity at different places.

Hope it helps