"Best" ending is a very ... loose term. Best is strictly which outcome best suits the objective of your own inquisition.
None... and i mean NONE of these people are redeemable in any shape or form (especially if you read the Masked Empire where you see what horrible cretins Celene, Gaspard and Briala are... each one of them are horrendous people, worthy of contempt and misery.. none deserve to live let alone rule).
Celene ordered the murder of peasants / servants in her own palace (Won't spoil the book by saying who they were, but they were innocents) all because it was "part of the game". She also torched the alienage, killing untold number of innocent elves just because some of them were rebelling. Not to mention the countless people she ordered assassinated (often using Briala as her personal assassin).
Gaspard, while having a "code" of conduct by the Chevalier code, also commits his fair share of atrocities and mass murders against innocents. He's a war monger and seeks to invade Ferelden as well. Also let's not romantice the Chevalier's as some have... their code allows and condones the execution of elvish peasants. In fact part of their Chevelier training before becoming a knight involves entering the Alienage and executing a few elves.. just to makre sure they're not squeemish about spilling blood... so.. ya, that "code" they have isn't worth anything.
Briala. Mass murderer. Has assassinated countless number of people, many of them awful people.. but also some who were just guilty of "being in the way" of the "the game" for Celene or the empire etc. In masked empire by the end of the book I was loathing her as much as Celene and Gaspard.. any sympathy i had for her was only because she comes from a much maligned and downtrodden people.
Awful.. all of them. The worst possible kind of people to have ruling / leading an empire.. they see life as cheap, expendable, tools for their own use. Even the mission in DAI, if you talk to them in length, you get a decent sense for how awful each of these people are as well.
So let's be cautious about tossing the word "best" around .. because no matter what the outcome, Orlais is ruled by either a single monster or band of monsters.