Apologies in advance for my reply being so long. Remember this is my opinion and is not meant to offend.
For the first few plays I chose Casandra as Divine. However I started to question my choice when Fiona said, “We cannot go back to way things were.” Even if Fiona did not say these words and someone else did, I still would have questioned my choice. Thus it is not the person who spoke it is the words.
These words are true.
In order to choose the best Divine it is not a matter of what is now, but how The Chantry came to the state they are in.
Going back a thousand years at the start is where the break-down began. The Chantry based its treatment on Mages on a quote from Andraste. The trouble is, we do not know the truth about Andraste nor do we even know she actually said this quote, let alone the same text and how she meant it to be interpreted. The Chantry chose to interrupt her words on how they chose too.
However it was not just Andraste’s words, it was also how The Chantry blamed Tevinter for her death. The fact is Andraste did die by the Tevinter’s hands, but she would not have done so if her husband did not betray her. Thus Andraste’s husband is to blame, not Tevinter, not the mages.
Sure, Andraste went to war with Tevinter, but that was based on how they ruled, how they treated people. Most wars happen when ideals do not mesh together.
Instead of The Chantry treating mages with respect and teaching others that mages are not the problem, they chose to instill fear into people. Fear of course is a good way to gain devotion and obedience.
Mages were locked away in towers and guarded by Templar overseers. This system no doubt worked for a while but every system breaks down if you do not maintain it. That is exactly what happened, The Chantry put on a grand show of maintaining The Circles, but did a poor job actually doing the deed.
Many Templars got away with far too much in how they treated their charges. If they were caught, the system would blame the mage involved. Of course their some situations to where the Templar is reprimanded but given no more than a slap on the wrist, in rare cases (such as Samson) the Templar is expelled from the Order.
Image if you will being a Mage and trapped in a Tower day in and day out. No matter how hard you study, no how you past your Harrowing, no matter if you use your magic to help people YOU are still evil in the eyes of The Chantry. That is like being convicted of a crime and being sentence to prison and no matter how hard you try to protest your innocence you will always be labeld guilty and a danger.
Some will say, "Well The Chantry is right, mages are a danger." No they are not. Mages are not dangerous, magic is the danger. But so is a man weilding a sword. A person who weilds whatever weapon they have be it a sword or magic can choose not to be a danger. Magic only becomes dangerous when a person chooses to use that power for purposes of evil.
Then we have the other side, "Well Mages can be possesed and turn into a demons." Yep they sure can. But we all learned in Dragon Age 2 it does not take a mage to be intisted by a demon's offer. Yes, they have a chance to become aboninations. But if every mage was that dangerous then every mage in Thedas would be an abomination. Not every Mage becomes a monster. Not every mage takes a demons offer. And The Harrowing is nothing more then a barbaric practice for The Chantry to show they have the power. Just because a mage passes his/her Harrowing does not mean they cannot become what The Chantry fears, what everyone in Thedas fears.
Thus it comes around to The Chantry being hypocrits. Blood magic is bad, but the Templars use it. Abominations are forbidden but Seekers are abominations. And there are many more issues that happen when The Chantry says it is forbidden.
So we then come to the question as the title has asked.
Cassandra is a good person, but she has a flaw in which she tries to change The Chantry but also tries to keep what once was intact. You cannot put a bandaid on a mountain sized problem and expect things to be as they were. The Chantry needs a clean slate, not a half washed plate.
Vivienne is thee worst possiable person to become Divine. She is not interested in the betterment of the people, she is interested in the betterment of herself. Yes, she gives the mages more freedom. But at what cost? How much freedom are we talking here? Not to mention she leashes the Templars firmly to her. This is a very bad idea. The Chantry tried to do this to the mages and failed. All this will do is start a countdown until the next war breaks out. Not to mention again, The Chantry states, "Magic is meant to serve man not rule over him." Yet here is a mage doing just that.
Lilly is the best choice (Unharden of course). Because she is willing to do what no one else will do-wipe the slate clean and start anew. This is what The Chantry needs, what Thedas needs. The Chantry was so mired in tradtion it could not see past all the cracks in the foundation. If you want to save something that is almost gone, you start over.
Am I in favor of the mages? Of course. But I play my characters diffrent. (My) Hawke was a firm beleiver in the mage rebellion, (my) Inquisitor is all for it but does not agree with how the mages chose to rebel.
However in the end you have to choose who is best for your game not what is best for everyone.
**Sorry for my bad spelling. Spell Checker is not working today***