That depends entirely on how you involve yourself in the war and who is left ruling the North at the end of the game.
-If Nilfgaard wins, it's basically a mix between the Roman Empire and Nazi Germany; Mages are kept in docile servitude under the threat of Death and Nonhumans are tolerated slightly more than they were.
-If Radovid rules, the North is "free." The Old kingdoms are absorbed into Redania, and Radovid's Witch hunters commit mass genocide against Mages and non-humans.
-The best ending for the Northern Kingdoms is arguably to let Dijkstra take over. He is brilliant, battle-hardened, and politically astute, while not being as cruel or tyranical as Emhyr or Radovid. I think this is the best situation for the Northern Kingdoms, but, you have to leave Roach, Vess, and Thaler to die in order to get it, which is a huge price to pay.
As for Ciri, I'd say letting her become Empress is the best situation for the overall world. But letting her become a Witcher is the best ending for her personally and for the core characters. And seeing as the crux of her story was her wanting to live her life as she chose to do so, without people using her for their own greater schemes, I say letting her become a Witcher is the "good" ending.