Elena, Riona, Nicole and maybe Josy and Quinn are the most natural looking girls. If I had to choose Elena's face is awesome, the textures and expressions are perfect, I think it's the most realistic model.Just off the top of my head I'd also go with Nicole (regarding a natural body design) and I'm probably forgetting a few others.
Bella and Jill are the opposite, I think they were the first models or they are made to appeal to patrons.
I hated Jill up to chapter 5 because of her unrealistic body and strange face but after the recital she is now my #1 alongside with Quinn, who was my favourite from the start. Her renders have improved too. The best and the worst girl for the MC but who cares, love is blindness.