Since we're all talking casually in this thread, I figured it'd kinda go into my rankings.
Little bit of context for my king character: I'm basing my king off of how I play my Dragonborn From TES V: Skyrim at a base that'd translate to a Nordic Warrior king who is a legendary dragon slayer and seems to have all of the magical powers of a dragon and is even said to be a dragon in human flesh. He's a devout follower of a christionesque warrior religion (givin the mismatch of high fantasy aspects I don't think real world Christianity would translate as well who prior to being king helping the helpless and slain the forces of evil. He ascended the throne after helping The *Totally not an Expy for a theoretical still existing roman empire* end a civil war that was tearing his country apart, and marrying the wife of the previously fallen queen. The problem is even despite his efforts to help keep the empire together using his influence on the council the empire itself is in its twilight years. The thing is he doesn't want his kingdom and people to become puppet vassal states to some foreign empire. So he wants to make sure his family gets into a high position within another more or better yet grow his own kingdom large enough to where they can strike it out on their own
Thing is he's not a fan of political intrigue. He'll if a decision does provide political benefits he'll certainly take it but he never does it strictly for the prestige or the political benefits. In fact even when the military governor of The Empire strongly urged my player character and the queen to marry he would not marry until he had proven to win the queen's heart. To him somthing like marrige is as much (if not more so) personal than political as it is political.
now with that context I'll give my reasoning for my top two choices and probably dwell into my other choices later when I have more time. it's getting late where I am currently.
Princess Aubrey: The fact that She's a grave robber and dungeon delver doesn't bother my character in the least. That's the kind of thing he does for both fun and to help the helpless and even when he was king he was still a part-timer he only stopped doing that sort of thing because he was soon to become a full-time dad. And now that his only child is full grown and soon to be married he may go back to doing that sort of thing part-time.
The part of her killing her husband would be concerning if it wasn't for the fact that A. given the circumstances surrounding her marriage the act was pretty understandable in my character's mind if not straight-up justified. and B. Unlike with her previous lover, she and my son actually have a strong bond and history with each other. They know each other like the back of thier hands and in my character's eyes would be perfect for each other so long as my son had tough enough skin to handle the nobles snarking behind his back. (Which he honestly hopes he does give how he tried to raise his son) Sure it wouldn't put them in a better position on the political front but it wouldn't put them in a worse position then they were either. He'd just have to find another way to accomplish his goals which he is both fine with and good at. Plus with his son already impregnating her, that means his son has got a surefire heir. they just need to legitimize him or her once they are born.
Initially. this was going to be his pick too and he was just going to go through all of the potential suitors with his son as a formal courtesy that was until they got to the very next suitor.
Princess Merida: My king and his son share a sort of spirited kinship with the people of Dun Broch (IE Scotland) and the surrounding Celtic and Angelo-Saxon kingdoms, in fact he feels more Kinship with them then his actual kin in Westeros. I will admit this is partial meta-gaming on my part. Playing CK2 i've found if you start off playing within the Brittanica area sometime prior to the year 1000, if you can get a foothold and establish a kingdom, The Empire of Brittania is one of the easiest to form.
Here it's even easier as not only would my son marrying Merida give my kingdom that initial segway/foothold, but the marriage is also backed by all 4 tribal kingdoms which with what I've researched seems to expies for Ireland, Wales, Scottland and possibly early England. This means I could establish an alliance with or even unite all 4 kingdoms peacefully and establish the empire of Brittana that much quicker which allows my kingdom to establish permanent independence.
On top of all that both Meredia and my son have much in common, they both have a thirst for adventure, and they are both very attracted to each other. The only Merida doesn't have compared to Aubrey is the personal history with my son and that gap can easily fill with time.
So essentially a marriage with Merida has potentially all the benefits of Aubrey with none of the drawbacks plus it puts my charecter's kingdom in the best position possible for what I want. Hence why my character calls it a 'Golden Union' rating it even above Aubrey.
Still it should be noted that my character would be perfectly fine with his Son marrying Aubrey if that's who his heart truly belongs to. He just thinks Merida would be the best choice overall.