IF I would vote from Geralts perspective, I'd say Yen. Obviously, since she is also my type of personality and since the connection they have in the books is undeniably strong (and since Ciri is not in question due to being his daugther and all).
If I would vote from my personal POV though I'd go with Ciri. Ashen Blond hair, looks badass, mad sword skills, magical powers, fast and acrobatic, strong-willed and those green eyes.... Yeah she might not be the definition of "hot" or "sexy" but I think she is the most attractive one all things combined (from my own perspective, not role-playing as Geralt).