Well, since others are giving their opinions, I'll weigh in on what missions I think we should take and which we should abandon based on this initial info. In descending order of what I feel is most important to take down to least important:
Renalta, Underdark, Amazons, Rheinfeld, and Liveria are the ones I think we should take. Goblin Holds, Free Holds, Imperium, and Tuleria seem like ones we should abandon for the moment.
Also here's my reasoning for those choices and that particular ranking, in a hider because it's kinda long.
For those I think we should take, first off Renalta feels like a no brainer must take situation. Letting Renalta get screwed over when it's supposed to be the core of the fight against the demons seems ludicrous to me, as does the idea of not getting angels on our side for the whole demon fighting thing, especially if there's a fucking archangel involved as Tempest said.
Letting the Underdark fall to the demons sounds like an awful idea. If there's awesome and powerful stuff down there that could empower the demons, and they have tunnels leading everywhere that the demons could use to invade without much warning, I feel like we should secure the Underdark to prevent that. Seriously, letting demons have underground paths to every other country is a baaaaaaaaaad idea.
Saving the Amazons sounds like a good idea to me, simply because losing the entire freaking civilization is a hell of a price to pay. They're supposed to be cool warriors, and letting demons frolic in the blood river might lead to bad shit. Oh, and if there's an entity that can destroy an entire bloody civilization over there, if we don't stop it now then it'll probably move on to try to fuck up other places. Best to deal with it now, imo.
Rheinfeld seems like a solid one to help, because all that anti-magic and experienced military folks would be really nice to have, plus the proximity to Renalta makes it a key place to hold.
And then I guess helping Liveria would be a good idea. Again, proximity to Renalta makes it a solid choice for protection, but they've got a nice long history of fighting dark and nasty creatures so their experience in that area would be nice to have on our side.
The first of the four I'd like to see us abandon is the Goblin Holds. It seems to me that other missions offer better long term rewards or have more dire immediate consequences for not taking them. It'd be nice to have Xixis and his dudes helping us, and Kasim has good in character reasons for wanting to take it, but I can't help but see the above five as more important.
The Free Holds don't seem like too bad a place to lose. Most of their military might comes from mercenaries, as I recall, and they're kind of a desert shit hole that's far off from Renalta. Honestly the only reason I rank it above the following two in importance is because it's actually got demonic shit happening now that we know of for sure, because of the succubus involved, so dealing with that demonic threat would be a nice start. If it were a non-demonic threat involved this would probably be ranked of lowest importance for me.
Tuleria kind of feels like a throwaway place to me, to be honest. Sure, it's a port to the rest of the world, but so what? They have those three NPCs who might be cool and powerful and stuff, but so what? Saving other places seem to offer more concrete benefits (like angels, anti-magic users, military might, keeping dangerous things out of demon hands, etc) and abandoning the Tuleria mission doesn't seem like it'll have as dire of results as other abandoned missions (like losing entire civilizations or losing powerful anti-demon resources). Seems the assassination going down will just throw Tuleria into civil war, which I think might be the second least severe of the potential penalties for abandoning a mission.
The Imperium doesn't sound extraordinarily helpful against demonic stuff. They recently got their shit slapped by the Free Holds using mercenaries and magic-based combat. I have a feeling that the invading legions of demons will be equal to (but probably better than) Free Holds mercenaries and will use plenty of combat magic as well. What use will the crusty old Imperium nerds be against demons if they got whooped by the Free Holds? I think the answer is "very little" or "none." I dunno what they've got going on for cool powerful NPC guys, but it sounds like their military isn't very well suited to the coming war so screw them. Also there does not seem to be any immediately obvious dire consequence to not taking the mission. Sure they'll be upset that we didn't help, and they probably won't get the soul gem thing, but an upset and gemless Imperium sounds far preferable to, well, every other obvious consequence for abandoning the other missions.