According to freud's theory of development, the best toy for an infant would be a:

Freud's first stage in his psychosexual theory of development is the Oral Stage.  This stage occurs between birth and one year.  At this stage, the baby or infant experiences his or her world through their mouth.  The baby is sucking, biting, and feeding. According to Freud, oral stimulation could cause issues later in life.  A toy that would provide oral stimulation would be considered detrimental (bad) to the child's development.