1. Who should rule Orlais?
2. Who should drink from the Well of Sorrows?
3. Who should become Divine Victoria?
These questions haunt me as I try to establish my personal canon. Seriously, I don't know what the hell to choose. And I love that. I like the ambiguity.
I don't know what kind of person you are or what kind of world you want to build.
1. What direction do you want the Orlesian Empire to go? Forget who you like personally, as they're all flawed, corrupt, and it hasn't seemed to work so far. So, what do you want for the Empire? Do you want it to be a more militaristic powerhouse a la Gaspard, continue its course of court intrigue but with support to the arts a la Celene, or give more rights to the city elves a la Briala? Personally, I want to see the elves in Orlais and Orlais-influenced countries gain a future for the first time in centuries, so to me the answer's obvious. Anything that'll get Briala in power.
2. Depends on how invested you are in gaining the secrets of ancient elven knowledge, how much your character trusts Morrigan with that knowledge, and/or how curious you are to see what the consequences will be like in future games. Personally, I play a Dalish who's very invested in recovering the People's lost history and lore, so she drank it. Other people have justified that they didn't trust Morrigan to be truthful about what she sees in the Well, et cetera.
3. Again, it depends on what kind of world you want to build, and/or which candidate you think would have the least backlash. Vivienne is easily the most conservative. She pretty much restores the status quo (Circles, Templars, etc), with the only real difference being that she's slightly more lenient on mages and slightly more restrictive of Templars. Cassandra is the middle option. While she restores the Circles and Templar Order (and likely Seekers), she reforms each one, roots out corruption, and imposes strict behavior monitoring systems to ensure the previous abuses that led to the war won't happen again. (At least not for now.) Leliana is easily the most radical "lefty liberal." She pretty much dissolves the Circles, reforms the Chantry to allow mages, elves, and men to join (whereas before the Chantry was pretty much limited to non-magic human women and the odd non-magic human lay brother), champions rights for mages and elves, etc.
While many people would support Leliana in theory, there are those who feel she pushes for too many changes too quickly to create a stable reign with lasting changes, or she herself is too dark and/or unstable to trust (since she spends most of the game teetering on the edge of moral bankruptcy), and so would prefer the more conservative and principled Cassandra (whom many feel makes enough changes to be effective but not enough to cause more revolts, and holds fast to her moral convictions).
However, the Divine is largely chosen based on decisions you've made and opinions you've expressed about the Chantry earlier in the game. For example, consistently siding with minorities like Mages and Elves, showing criticism of the Chantry, etc. often leads to Leliana becoming Divine, sometimes even if you verbally support another candidate near the end. So, from what I hear, a lot of that is a little out of your hands.
Again, I'd start with asking yourself what kind of world do you want to build? Which factions do you want to support? It can make it much easier to choose who rules Orlais and who becomes Divine, based on whose vision you support.
So, almighty BSN, what are your preferred answers to these questions?
I play a Dalish Keeper's First who consistently sides with Mages, Elves, and recovering ancient elven history and lore.
1. Briala because I want the elves in Orlais and Orlais-influenced countries (which is most of Southern Thedas and Antiva) to have a future.
2. Again, I play a Dalish Keeper's First who greatly values knowledge and recovering her people's lost history and lore, so she drank from the Well. "It's my heritage."
3. My character didn't make an official stance regarding who she supported for Divine, so the Chantry elected Leliana. I personally prefer Leliana "softened" though (by completing her peronal quest and have her realize she shouldn't let her ruthless assassin side consume her), because she directly addresses the critics and urges them to put their differences aside and work together for the greater good, rather than sitting quietly if her quest goes uncompleted, or assassinating them all if "hardened."