Where is the best place in a circuit to find out absolute zero when measuring voltage

Between the last component of the circuit and the source.

Explanation:  In a closed circuit, the Kirchofs law says that:

∑vₙ = 0

This means that the summationof all the voltages is equal to zero, wherein the source or fountain, the voltage is positive, and in each component of the circuit (resistances, capacitors, etc) the voltage is negative.

Suppose you have a source that produces a voltage V and two resistances, this law says that the voltage V will vanish completely after passing by the two resistances.

This means that in the segment between the last component of the circuit and the source the voltage must be equal to zero, and that is the place where you will measure the absolute zero.