In my playthrough I thought I could trust Celene more than Gespard, but that was due to pretty specific situation - Celene was someone who would want to make some (good) reforms but she's also cowardly and when things seem to go sour, she does ANYTHING to ensure that she stays in power. However, with Gaspard out of the picture and powerful Inquisition on Orlais' doorstep with an elf in charge (elf who outright demanded recognition for the fact that he is, indeed, an elf - that's how I understand the declaration of ideals when assuming position of Inquisitor in Skyhold), Celene has an ally that can keep her on her throne AND turning on elves in this situation seems like a very dumb thing to do, especially since Briala was made Marquise of the Dales, which means that in a couple years time she would have a real army, not just a bunch of kitchen staff with knives.
Basically, Celene can do reforms (she seems to want to), be all lovey-dove with Briala (oh, darling, you don't hold a grudge that I killed your parents and massacred insurgents in Halamshiral, do you? This relationship is sick, good thing I like neither of those two b*tches) and feel relatively secure on her throne. In the same time, turning on elves OR on Inquisition puts her in a very unfavorable position. So, basically, she has lots to gain on an alliance and nothing to lose on turning on me.
Now let's see Gaspard. He wants to invade Ferelden - and Inquisition is friendly with this nation too and doesn't really want a war, especially with their HQ pretty much on the border between them. Gaspard doesn't like elves - and there is this freaking elf inquisitor. Gaspard is also all for Orlesian imperialistic pride, which means that his internal position is likely to be much stronger than Celene's (especially with Celene reunited with Briala). Unlike Celene, Gaspard doesn't really need Inquisition and may even have serious disagreements with it concerning things that are important to him (going to war to conquer something, that is). And this is why I trust Celene more than Gaspard - not because she as a person is more trustworthy, but because she doesn't have much reason to betray me (and definitely has some to remain my friend) while Gaspard's conflict with Inquisition is something that I believe to be nigh-unavoidable. At least with my Inquisition that is led by an elf and doesn't really like its pet kingdoms conquering one another
As for Gaspard with Briala, I said why I believe it to be a disaster waiting to happen.
To sum things up, I think there are at least two rankings, both possibly changing a bit depending on who the Inquisitor is. It goes like that:
A) If your priority is Orlais as a strong nation, you want:
1 - Solo Gaspard, he seems like someone that could really make it a strong nation, even though he wants conquests that aren't really good for Orlais, especially at this point.
2 - Solo Celene, if you support her, she can push for reforms that Orlais really needs. Looking for new wars isn't a good idea, really. However, Celene is too reliant on the Game and that's why she gets only second place; and why she really needs your support
3 - Celene + Briala, they also push for reforms. Their position is weaker than Celene's if she's alone and regardless of what political correctness might claim, the fate of the oppressed doesn't really make the nation as a whole worse-off, so effort put into improving situation of elves could be spent with better (for Orlais) effects elsewhere. Still, that's a solid option.
4 - Threesome. It can be very beneficial for Orlais in the short run, but this triumvirate is bound to break. The best case scenario has either Celene or Gaspard taking full power at some point. Worst case scenario is another civil war, possibly worse than the last
5 - Briala with Gaspard as her puppet. I believe that is the worst option. It can't work. Gaspard making Briala's reforms will be hated, but he'll hate her and Inquisition, and elves too. Briala has good chances of getting drunk on power, pushing for elf-related reforms and pretty much neglecting other issues. I think those two would be just plainly dysfunctional, and then finally Gaspard will break free, kill Briala and enact vengeance - I don't see him making any deals with Inquisition after Briala is dead. Even if his internal position will be very weak by that point, even if he would really benefit from maintaining good relations - I don't believe he would have enough composure to just "forget" hatred born of years of humiliation.
If we look at the situation from different angle, namely: elven angle, the order is different.
1. Celene + Briala - this tandem with Inquisition's support can really make a change and most likely will want to make a change. Backed by Inquisition, Celene most likely won't backstab the elven cause since the main reason (fear of losing power) shouldn't really apply. Even if he does turn on elves, it likely won't be with malice (making it slightly less disastrous).
2. Threesome - they will likely improve the lot of elves (if just a little bit) and whoever remains standing after the truce crumbles won't really have much reasons to hate elves. And will have lots of other things to think of. Basically, this is a low-gain scenario but the gain will likely last.
3. Celene solo - the Empress is supposedly friendly to the elven cause, but hardly trustworthy and her power comes first. With Inquisition's support she will likely provide some small reforms... but that's not certain. An iffy choice with possibility that things will improve - but only a little even in the best case scenario.
4. Gaspard solo - This guy doesn't really like elves, doesn't respect elves, likely sees them somewhere between a draft horse and a human. A bit below a good riding horse, actually. With him, nothing will change
5. Gaspard as Briala's puppet - yeah, I hate this option. This path is likely to give quick, noticeable improvement and then a great plunge with elves ending up worse off than they started. Reforms made by emperor who doesn't want them, forced to it by means of blackmail... That just can't turn out well.