There is no single best plywood thickness for every design. A thin sheet may work well for a small model, while a long freestanding part needs more stiffness. Consider the shape and how the part is supported first.
Size and support work together
A panel secured around all its edges has different requirements from an equally large part resting on two points. Long narrow parts bend more readily than compact shapes. A reinforcing rib or additional support may help more than simply increasing thickness.
Compare wood species as well as thickness
Birch, poplar and basswood plywood differ in weight, construction and appearance. The same nominal thickness does not mean two sheets will feel the same. Compare individual products in the plywood collection.
For a flatpack, the weight of all parts matters. Greater thickness also increases package size and changes the length of joints.
Adapt slots to the chosen material
Actual thickness can differ from nominal thickness. For snap fits and press fits, make a small series of slot widths. A joint that works in birch plywood may fit differently in poplar.
A sample or a prototype?
A sample helps you assess colour, grain and edge appearance. To evaluate bending, stability and joints, use a prototype with the relevant dimensions. Then select the thickness for the complete set. Read more about designing joints.