Okay, I finished a marathon of playthroughs, and I think my favorite ending is Mora/Heidi/Athena - Mora is probably my favorite, and I can stand the thought of not helping her out of her situation. Playing Athena without Heidi leads to scenes where Heidi looks sad, which is heartbreaking. And going for Heidi without Athena breaks them up, which is also super sad.
I like how Zenda and Valeria were added as monogamy only options. I think of the solo endings, though, Heidi's and Mora's are my favorites.
Overall, super entertaining game. The best games are the ones with girls who are entirely distinct, and each of the girls here absolutely feels like their own person.