A Negima fan blogger surveyed the popularity of Japanese voice actresses by Google Trends. The popularities were quantified according to how frequently a given actress was searched on Google from January till May 2009. The scores are standardized assuming the popularity of Tomatsu Haruka as 1.0.

*1, 6.20 Hirano Aya
*2, 5.70 Mizuki Nana
*3, 2.32 Tamura Yukari
*4, 2.24 Chihara Minori
*5, 2.12 Sakamoto Maaya
*6, 1.94 Nakajima Megumi
*7, 1.60 Ikezawa Haruna
*7, 1.60 Kugimiya Rie
*9, 1.49 Horie Yui
10, 1.48 Inoue Marina

11, 1.44 Kobayashi Yuu
11, 1.44 Tanaka Rie
13, 1.12 Noto Mamiko
14, 1.11 Koshimizu Ami
14, 1.11 Shintani Ryoko
16. 1.07 Itou Shizuka
17, 1.04 Kitamura Eri
17, 1.04 Goto Yuko
17, 1.04 Sato Rina
20, 1.00 Asumi Kana
20, 1.00 Tomatsu Haruka

Below 1.0
Source: Catmania blog

Update
Voice Newtype magazine carried out a regular vote on the most popular voice actor/actress. The ranking and the numbers of votes are as follows:

Voice Actress
*1, 541 Hirano Aya
*2, 451 Mizuki Nana
*3, 313 Kugimiya Rie
*4, 270 Tamura Yukari
*5, 257 Tomatsu Haruka
*6, 227 Chihara Minori
*7, 223 Horie Yui
*8, 211 Paku Romi
*9, 186 Inoue Marina
10, 173 Saiga Mitsuki

Voice Actor
*1, 728 Kamiya Hiroshi
*2, 483 Ono Daisuke
*3, 476 Sugita Tomokazu
*4, 262 Nakamura Yuichi
*5, 259 Suzumura Kenichi
*6, 241 Fukuyama Jun
*7, 230 Yoshino Hiroyuki
*8, 228 Sakurai Takahiro
*9, 221 Miyano Mamoru
10, 216 Suzuki Tatsuhisa

Source: Scan of Voice Newtype