If she doesn't end up losing it the MC won't be able to return it to her in a touching true-love scene type thing. "Oh MC, you remembered...wanna fuck?" or something, you know, well written. You get the idea.
As for favorite, I assume you mean out of all the (main) girls?
1- Rae, always has been, probably always will be, but...
2- Lyriel is making a strong push. I disliked her initially, quite a bit actually. But she really grew on me. Plus, you know, smoking fucking hot blonde elf.
3- Cait. Cute, "innocent", mousey redhead with big tits. If 1 and 2 weren't super special she'd easily be #1.
4- Naomi. I like her personality and all but...she bench-presses the bench. If you're into that cool but it's not my thing. Totally go out drinking with her though.
5- Yona. Meh
Honorable mentions : Annie (yes please!) and Jenny