One of the few things that TPM got right was showing Anakin as a slave but as a slave without ambition (other than seeing the stars).
As his ambition grows he remains a slave but becomes more and more frustrated by it.
He is a slave to the Jedi, their disciplines, rules and the special part he is meant to play in their destiny.
He is a slave to emotion and to his perception of the future (this is the weakness Palpatine uses to cage him).
Then he becomes a slave to the dark side (effectively the property of his Master) and his ambition to remove Palpatine's heel from his back.
The only time he isn't a slave is when he decides to place the safety of his son before that of himself.
Anakin redeemed is a stronger man than the weak slave Darth Vader because he is for once his own man.
It's a shame that Lucas potrayed the character so poorly in the prequels because it has such potential as piece of drama but that's a discussion for another place.