Gotta be the Tomboy and Mya, the beach girl chilling by herself. For the Tomboy, yeah it's messed up that we ruined her life like that (which got expanded upon in Breedtown 2) but there's that JLullaby RWBY design aspect to her that makes her so irresistable. Mya, on the other hand, had that mechanic where you could keep coming back to her until you bred her a certain number of times. I know that some gals in the second game also had a similar thing going for them but it doesn't quite hit the same, personally.
The third game's... an interesting direction, I guess.