The following information comes from an official Square-Enix poll...

1. Lightning from Final Fantasy XIII
2. Aerith from Final Fantasy VII
3. Yuna from Final Fantasy X
4. Tifa from Final Fantasy VII
5. Celes from Final Fantasy VI
6. Terra from Final Fantasy VI
7. Serah from Final Fantasy XIII
8. Garnet from Final Fantasy IX
9. Faris from Final Fantasy V (tied for 8th)
10. Rinoa from Final Fantasy VIII
11. Rydia from Final Fantasy IV(tied for 10th)
12. Selphie from Final Fantasy VIII
13. Rikku from Final Fantasy X
14. Beatrix from Final Fantasy IX
15. Yuffie from Final Fantasy VII (tied for 14th)
16. Ashe from Final Fantasy XII
17. Rosa from Final Fantasy IV
18. Eiko from Final Fantasy IX (tied for 17th)
19. Agrias from Final Fantasy Tactics (tied for 17th)