Rikku if its all pure looks and Rinoa if personality is involved. Rikku is self explanatory why she is sexy and Rinoa just has that playful, caring, but not just eye candy personality that is just SO attractive.
I think most people voted Rikku in because of that one cut scene too. Thats what really swayed us all I think.
EDIT: Honorable mention Fran. Agreed to have the finest ass.