I'm not sure which Divine is the absolute best. I can see they are three interesting characters with very valid views of the matters in South Thedas. I like all three of them in some ways and I think they have some evident flaws (Viv is a tyrant, Cass dislikes politics too much, hard Leli makes bloodshed her sport and enlightened Leli is too soft for such radical changes). I have no honest idea of which one will be objectively the best, and endgame slides have been notoriously misleading in past games.
My perspective is a role playing one, and from a role playing point of view: each one of them is the best for some of my different characters. I find this discussions about who is the best Divine a little improductive and repetitive.
By the way some tiny part of me thinks that Mother Giselle could have been good Divine material. She has an awful moment, her role in Dorian's quest, that I'm still not sure if it's plain homophobia or irrational hate to Tevinter, but what she did when the Inquisition was in the lowest moment after the attack on Haven was the best an intelligent cleric could do. That's the way I want a religious leader to act: gather the faithful when they need to, rise their spirits and make them work for the common good.