Kingdom of Renalta: Home field, we want to have this place set down and secure. Anti-demon units if we secure angels, an archangel amongst them
The Imperium: Strong fighting force, scant on magic, quest deems high potential loss for a named NPC that could weaken Imperium, artifact could be strong in demon hands
Kingdom of Rheinfeld: Next door neighbor to Renalta, strong point for demonic invasion, high amount of anti-magic/potential anti-demon units. IN THE MIDDLE OF CIVIL WAR
Kingdom of Liveria: Next door neighbor to Renalta, fairly isolated. Could spell loss of the civilization as is and destabilization if the princess is lost. 
The Goblin Holds: Barrier between the Free Holds and Renalta, strong point to control, strong fighting force
The Free Holds: Strong point for demonic invasion, very prone to corruption area, but more outlier. Could stand to lose if we have buffer (Goblin Holds). Major Mercenary export
The Underdark Coalition: Isolationist country, outlier. Could stand to lose, but offers a lot of high-strength units and potential equipment improvement. (Mechanists, Dwarves, Dark Elves), high point of invasion potential
Kingdom of Tuleria: MAJOR PORT to exterior world. Destabilization of the power can remove high power NPCs or throw the country into civil war. 
The Amazons: Outlier, would lose the civilization. Would suggest against loss given their proximity to the blood river. COULD BE MAJOR DEMON INCURSION SIDE-EFFECTS TO LOSE

Ones I would think are less prime objectives: Imperium, Liveria, Free Holds, Underdark

Personally, I have a major character reason to why I want to go with the Renalta mission. I also have a major character reason why I'd like to go to the Imperium. But from what I can see, these are the areas that can do taking a small hit and offer less vital resources.