According to poll conducted at the Biglobe website, anime fans voted for Magical Girl Lyrical Nanoha as best magical girl themed anime series.

Lyrical Nanoha is a story of a normal third-grader, Takamachi Nanoha. Surrounded by a loving family and caring friends, Nanoha is content with life. However, one day she hears a voice in her head which leads her to an injured ferret guarding a red gem. After rescuing the ferret, named Yuuno Scrya, he gives her the gem and awakens Nanoha’s magical powers, asking that she use them to fight alongside him.

Here are some mahou series anime included in Top 20

Rankings

1. Magical Girl Lyrical Nanoha
2. Cardcaptor Sakura
3. Puella Magi Madoka Magica
4. Ojamajo Doremi series
5. Bishoujo Senshi Sailor Moon series
6. PreCure series
7. Kiki’s Delivery Service
8. Shugo Chara! series
9. Akazukin Chacha
10. Magical Princess Minky Momo
11. Creamy Mami, the Magic Angel
12. Kamikaze Kaito Jeanne
13. Fushigiboshi no Futagohime
14. Mermaid Melody Pichi Pichi Pitch
15. Sugar Sugar Rune
16. Full Moon o Sagashite
17. Kaito Saint Tail
17. Himitsu no Akko-chan
17. Majokko Megu-chan
20. Corrector Yui

 Poll was conducted between May 31 to June 7 with a total of 3,174 respondents voted.

[Source]