Okay, I don't want any spoilers to ruin things for people but in your experience, what makes an ending 'work' in a YA adult? Fine to list titles (but not the ending itself).
It has to answer most (if not all) questions. It should tie everything together in a nice little bundle. The climax should make the reader wiggle in their seat, be riddled with tension and stakes. The ending should satisfy a reader in the sense that they believe the problem has been resolved.
Not to be dark or twisted, but my favourite endings are the ones that aren't happily ever after. From time to time I like them, if it fits and it isn't just there because an author can't let one or two of their characters go.
I usually like my endings with a huge life altering event. Such as maybe one of the team members get killed and dragged away to the hostages, implying that maybe these characters failed at their attempt. Maybe they won the war but lost many people during it, maybe one or two being a primary character.
To me that is realistic in a fantasy or horror story in my opinion. Not everything is candy canes and lollipops.