A positive test charge would have the least potential energy if it were placed at the center of a sphere of negative charge. The positive charge would be attracted to the negative charge and would be held in place by the force of attraction.
Other related questions:
Q: Where is the least potential energy?
A: The least potential energy is at the bottom of the potential energy curve.
Q: Do positive charges have potential energy?
A: Yes, positive charges have potential energy.
Q: Does a positive charge moves from high to low potential energy?
A: No, a positive charge actually moves from low to high potential energy.
Q: What is the potential energy of the test charge?
A: The potential energy of the test charge is the energy required to move the charge from its current position to the point where it would be at rest.