My two-cents for why I think Vivienne is the best Divine:
First - I dont believe mages should be free, across the series they did so much s*** and when they do their s*** it can last centuries or it will cause a lot of damage. The Blight was caused by 7 magisters, Werewolf curse was caused by Zatrhian, Connor raised the dead, Anders killed hundreds with his explosion, Solas gave his orbs to Corypheus who used it to open the Breach, etc. To me those death could be all prevented if they were being watched, so I will never agree with mages being free or watching themselves, look how good Fiona's rebels did watching their own, every dalish clan we met, have mages fuc*ing thing (Zatrhian and the Werewolf curse, Merrill/Marethari/Audacity debacle, Velanna on a killing spree, the clan that summoned Imshael and the in Inquisition a dalish mages kill himself with bloodmagic) and I dont even need to mention Tevinter (blood sacrifices. Blights, Lyrium marks, etc), because of this I will never agree with Leliana.
Second - I love the Templar Order, they are know as the "Champions of the Just", they are the first and the last line of defense of the Chantry, the common folk revere them for their feats, thay are responsible for bringing down Abomination, they were at the vanguard when a exalted march was called to fight against the Blights, the elves of the Dales, Tevinter empire and the Qunaris. They sacrifice their lives for the better of the people, they ingest Lyrium knowing of what it will do to them, I respect them and what they represent, I dont want them gone, so again Leliana is out of question. But Im not blind of their abuses, I think they should pay for their crimes and there is a need for better oversight to prevent abuses.
So its come down to two candidates, while I like Cassandra, I disagree with her standing with the mages ("let the mages govern themselves"), they proved they cant, and she is not good at politics, the Divine is a political job besides being a religious one.
Vivienne is perfect to me, she put mages back into the Circles, giving them more freedom and responsabilities, and leash the templars into her, preventig abuses from happening or templars doing what they did in the Hintherlands. In my ending (with the templars conscripted) some templars went back to the Order, those who went back know they will serve a mage, they are not mage haters, and neither will be the new recruits (if they hate mages, why serve one?),so its a time for a new Templars Order, one that will protect mages and mudanes alike from the danger of magic.