The form of sugar transported through phloem is

The form of sugar transported through the vascular tissue, phloem from the source to the sink is sucrose.

Sucrose contains more energy than glucose so it is more energy efficient, both in its transport as well as in its storage. In addition to that sucrose is non-reducing so it does not undergo any kind of reactions with other molecules so can be transported in its original form other than glucose which is reactive.