UV Unwrapping
Model should have unwrapped UVs so textures can be applied to it. Even if the model is intended to be used with simple colors, it is still best practice to have usable UVs so it can be used in more ways. In some engines and software a model without any UV mapping might not be rendered or displayed correctly and create technical problems and limitations.
No overlapping per UV island
All UVs islands should be properly laid out without there being any overlapping parts within each island. This can also happen if UVs are not properly welded and have seams within them that are not present in the actual geometry. Different UV islands can be stacked or overlapped, there should just not be overlapping parts within the island itself.
No texture stretching
UVs should be unwrapped in such a way that there is no visible stretching when textures are applied. This can generally be achieved by making all UV shells as straight as possible.
No obvious/unnecessary UV seams
UVs should be made to have as few seams as possible, any UVs that are unavoidable should be placed in areas where they would not be obviously visible, like along edges or in locations that are not very visible.