There is no easy/short answer for that.
Some things to consider:
-Insulation requirements (I mean from a safety perspective if applicable, this affects insulation thickness and margin to get certain minimum creepage path)
-Leakage induction (interleaving gives lower leakage inductance).
-capacitance between prim and sec winding.
-presence of air gap (avoid thick wires close to the airgap as this increases eddy current loss in the wire).
-is it a transformer only, or does it need to store some energy (as in a flyback converter, you need airgap, or very low permeability material).
-interference from primary to secondary (make sure that the cold side of a multi layer winding sees the other winding).
Just primary and secondary on top of eachother is easiest, you have the lowest capacitance, but the largest leakage induction.
sec-prim-sec (interleaving) gives more capacitance, but significant reduction in leakage inductance. It is best that both windings have same width, you may use more wires in parallel for the primary so you get good copper coverage.