Go back and look carefully at the salutation of the angel Gabriel. This salutation itself ("Hail, full of grace!") makes a very clear statement about the unique nature of Mary. In general terms, grace is the means by which humans are granted salvation by God and saved from the effects of original sin. Grace by definition is the result of Christ's Redemptive mission, and Old Testament figures did not enjoy any fruits of this Redemption while they lived on this earth (even those who were greatly favored by God).
In this context, this statement by Gabriel that Mary was "full of grace" can be seen as a remarkable acknowlegement that she had already been given the benefit of Christ's Redemption before He died on the cross and rose from the dead.
From The Meaning of Kecharitomene: Full of Grace (Luke 1:28)
"Kecharitomene" ('Full of Grace' in the Greek text) means "having been" or "have already been" graced.
It also shows a completeness with a permanent result
Precisely, one who does not have salvation assured CANNOT be "full of grace". Moreover, a soul stained with sin can only obscure the Lord, it CANNOT magnify the Lord. And finally, if she is blessed AMONG women, that means ALL women and Eve WAS created in a sinless state.