Which syrup to choose for winter feeding?

The quality of syrups used for winter feeding of bees is very often at the centre of lively discussions.
What should be the main qualities of a syrup:

• A composition close to that of honey
• Easily assimilated by the bee
• Requiring minimal energy for conversion and storage
• Good solubility (no crystallisation)
• Good stability (no fermentation)
• High purity and appropriate viscosity