Fate/Zero is the clear winner in Japan -- although Shana may be a close competitor. It's not possible to tell how many of its 242 threads of posts are for earlier series.
Gundam AGE seems to be far more interesting to posters abroad than it is in Japan. Horizon and Persona 4 are a bit more popular in Japan than overseas.
Anyway, here are the figures:
2channel (thousand-post threads)
|Last Exile Fam||20|
|Kimi to Boku||6|
|Last Exile Fam||678|
|Kimi to Boku||122|
This method is not infallible. Discussions of some shows -- of Guilty Crown on AnimeSuki, for example -- have as many negative posts as positive ones.
And I wasn't able to tell how many threads for some continuing shows were for the original series rather than the new one. That's true for Shana, Working, and Mirai Nikki.
By the way, Haganai is the nickname for the show Boku wa Tomodachi ga Sukunai, which is popular both in Japan and beyond -- for good reason.