Manga Anthology Magazines
 

I am looking for advice from anyone who knows a bit about the numerous weekly manga anthologies that appear in Japan (and numerous indeed!).

I did a bit of experimenting on my own and picked up the latest issue of Afternoon (アフタヌーン)
http://en.wikipedia.org/wiki/Afternoon_%28magazine%29
and then I saw on the shelves Chika Umino's March Comes in Like a Lion on the front cover of Young Animal (ヤングアニマル) so I picked that up too - Only to find out that the main draw of Young Animal was Actually topless girls, although the drawings were very nice.
http://en.wikipedia.org/wiki/Young_Animal


So my question is, which are the Go-To-Manga-Anthologies for these respective demographics? Seinen, Josei, Shounen, Shoujo. Which are the mainstays of each of the publishing companies like Hakusensha and Kodansha?

You can check this site for many many manga magazines, manga anthologies, comic imprints, light novel labels, and more:
http://www.comipedia.com/

If you mean what manga magazines that bring the most profit to its companies, then for Kosansha it is its shounen manga magazines: Weekly Shounen Magazine (about 1,700,000 readers) and Monthly Shounen Magazine (about 940,000 readers).

As to Hakusensha, their magazine readership is far from reaching Kodansha's readership, their biggest manga magazine readership is their shoujo manga magazine Hana to Yume (about 260,000 readers), which is smaller than Kosansha's shoujo manga magazine Nakayoshi's readership (about 340,000 readers).

However, I must add that though the circulation numbers of Hakusensha's magazines are relatively low, their shojo titles are the ones to beat. Still, it's probably best to wait for the tankubons.

For josei-lite, I think Cookie is ok. Although I read only Nana from the magazine in question, the other series doesn't seem to disappoint.

Young Animal is a seinen magazine. So, yeah.

It's true, people would be well advised to know Sangatsu no Lion is in no way representative of the stuff in Young Animal

The seinen mainstay would be the Big Comic line or Morning, but the fact that you bought Afternoon would suggest to me a somewhat different taste, so if you're looking for something for yourself, the best way to go about it would probably be to give us some idea of what you're into (so, like, what Afternoon stuff do you like?)