The 2channel reader software I'm using now (Chaika, a Firefox add-on) shows the current posting rate for each thread on the big Japanese discussion board. So we can easily see which seiyuus 2channelers most like to talk about right now. (mouseover photos for captions)
Mizuki Nana, Horie Yui, and Ochiai Yurika continue to lead the list, as it seems to me they and Tamura Yukari have for several years. Nana-chan for her singing more than her seiyuu work. Hocchan for her many, many moe roles. And Yurishii mainly for her emotional personality and the difficulties she has in life, which she describes so openly in several blog posts a day.
The 2channelers rate Asumi Kana (Yuno in Hidamari Sketch) surprisingly highly. And you can see the young seiyuus from K-On -- Toyosaki Aki, Hikasa Youko, and Satou Satomi -- surging toward the top right now. And the seiyuus from Suzumiya Haruhi continue to be discussed -- especially Hirano Aya and Chihara Minori, but Gotou Yuuko as well.
Only two guys make this list of the top 25 names or so: Kamiya Hiroshi and Fukuyama Jun. That is, if you don't count a male seiyuu who is known to belong to the somewhat cult-like Buddhist group Soka Gakkai and has thereby earned an anti- thread from the 2channelers.
The seiyuu with the most anti- posts, however, is still Hirano Aya, whom many Japanese otaku love to hate. But Nakajima Megumi is catching up to her. In both cases, the antis think they are poor seiyuus and are enraged by people praising them, and ridicule anything the two seiyuu/singers say or do.
Anyway, here is the list. A completed thread represents 1000 posts:
Mizuki Nana: 462 threads total, posts coming at the rate of ten an hour.
Horie Yui (345) 6/hr
Ochiai Yurika (247) 6/hr [Konomi in ToHeart2, but mainly for her plaintive blog]
Asumi Kana (62) 5/hr [Yuno in Hidamari Sketch]
Nakamura Eriko (59) 5/hr [games, esp. Haruka in Idolmaster]
Tamura Yukari (522) 4/hr [Nanoha, etc., etc.]
Toyosaki Aki (27) 4/hr [Yui in K-On]
Hirano Aya (168) 3/hr [Haruhi, etc.]
Chihara Minori (153) 3/hr [Yuki in Suzumiya Haruhi]
Kaneda Tomoko (48) 3/hr [Chiyo in Azumanga Daioh]
Hikasa Youko (11) 3/hr [Mio in K-On]
Imai Asami (81) 3/hr [games, esp. Chihaya in Idolmaster]
Satou Satomi (6) 3/hr [Ritsu in K-On]
anti-Horie Kazuma (5) 60/day [Kaname in Natsume Yuujinchou, criticized for his Soka Gakkai connection]
Tomatsu Haruka (20) 60/day [Nagi in Kannagi, etc.]
anti-Nakajima Megumi (39) 58/day [Ranka in MacrossF]
Kamiya Hiroshi (35) 56/day [Natsume in Natsume Yuujinchou, etc.]
Taketatsu Ayana (6) 56/day [Azusa in K-On]
Gotou Yuuko (77) 55/day [Mikuru in Suzumiya Haruhi]
Asano Masumi (138) 51/day [Risa in Hayate, etc.; has long-running weekly radio show]
Mizuhara Kaoru (19) 50/day [Misao in Lucky Star]
Fukuyama Jun (24) 50/day [Lelouch, etc., etc.]
Katou Emiri (31) 49/day [Kagami in Lucky Star, etc.]
anti-Hirano Aya (65) 39/day
Takahashi Chiaki (59) 39/day [Mitsuki in Kimi ga Nozomu Eien]
The list could be longer, but I just took what fit on one screen, about 25 names.
Seiyuu fans in Japan see and hear seiyuu a lot more than we do outside Japan, and in more settings: TV, live events, radio, magazines, games, etc. A seiyuu's current work is not necessarily the main thing in the fans' minds.