Best Ciri ending? Hmmm...
The witcher ending is a good one, and I think it fits a little better with Triss romance, because it becomes easier for Geralt to check on Ciri if necessary. When he romances Yen, they seem to isolate themselves, and the ending itself conflicts with Geralt's behavior towards Yen: it's Yen who keeps telling Geralt to report on Ciri to Emhyr and the witcher ending is a direct result of Geralt not taking Ciri to him. Well, if you romance Yen, you're far more likely to listen to her advice, right?
But I just don't like Ciri as a witcher. She doesn't have any of the "tools" of the witcher at her disposal, just the silver sword, and being a witcher is much too easy work for her, especially if she learns how to utilize her powers. The whole point of a witcher is to be someone who can go toe-on-toe with monsters but still risks a lot. Nekkers are already easy for Geralt, imagine for Ciri!
So I think the empress ending could be the best. It gives Ciri a fixed location, works with either Yen or Triss romance (although Yen is preferred in this situation), and makes the Nilfgaard victory not much of a sacrifice as it is in the ending where Emhyr is the ruler. A North under Nilfgaard ruled by Ciri is in the best position to prosper after the war.
Ciri dying means you failed at being a good father so it's the worst outcome by far. IMO it only makes sense if you play "evil Geralt": ignore Triss' plea to help the mages, break up with Yennefer, don't help either an Craite sibling, ignore Radovid quest, and always go after the money in small quests.