Macross Frontier and probably Code Geass top the list of most posted-about new shows on 2channel. Macross is at almost 87,000 posts. The most popular new shows in TV ratings, however, are Blue Dragon 2 and Net Ghost PiPoPa. I'll get to TV ratings later in the article.
Unfortunately, it's hard to know the 2channel popularity of Code Geass, since the threads have just continued on from the first series. It is now up to 578,000 posts, overall.
Coming after Macross's 87 threads (86,000+ posts), way down at 20 threads, is Zettai Karen Children, which apparently took over Hayate no Gotoku's popularity when it took over its TV timeslot.
Kurenai, Soul Eater, and Library Wars follow, at 13 threads each. ToLoveRu has 12.
Kamen no Maid Guy is at 8, and Blassreiter and Kanokon are at 7 each. Kyouran Kazoku Nikki has 6, and Nabari no Ou and Wagaya no Oinarisama both have 5.
Allison to Lillia and Vampire Knight are at 4, and there is a long list at 3: Golgo 13, Itazura na Kiss, Mnemosyne, Neo-Angelique Abyss, Nijuumensou no Musume, Penguin Musume, and Special A.
Chi's Sweet Home and Crystal Blaze are at 2, and Bus Gamer at 1.
It's hard for me to tell about XXXHolic 2 and Da Capo 2 SS, since they both may have just continued threads from their first series, as Code Geass seems to have. But I don't have access to the 2channel archives to find out. XXXHolic is at 20 threads and Da Capo 2 SS at 9.
This list simply shows how interesting shows are to people who post on 2channel. It is quite different from general popularity as represented in TV ratings. For those, check out Siegel Clyne's posts in the "Anime TV Ratings in Japan" thread in the AnimeSuki forum.
In TV ratings, family anime like Sazae-san and Chibi Maruko-chan had 18 and 14 percent of the TV audience in their time slots in the week ending April 20. Anime like One Piece, Detective Conan, Gegege no Kitaro, and Naruto were around 6-8 percent.
Then there was a group around 5-6 percent: Major, Precure, Keroro Gunso. And Gintama was just under 5 percent of the audience.
Between 3 and 4 percent came Blue Dragon, Bleach, Shugo Chara and Net Ghost PiPoPa. As far as I can see, Blue Dragon 2 and PiPoPa were the highest-rated new shows.
Then, just under 3 percent of the audience, we have a batch of new shows: xxxHolic Kei, Toshokan Sensou, Golgo 13, ToLoveRu, and Code Geass.
Between 2 and 2.5 percent were Onegai My Melody Kirara, a rerun of Prince of Tennis, Macross Frontier, Soul Eater, Real Drive, Itazura na Kiss, the late show of Soul Eater, and Zettai Karen Children.
Figures are not available for all shows. And some shows will be missing from the 2channel list, as well, since I only took shows that were among the top 100 recently posted about.
EDIT: Here are a couple of sites that give TV ratings. Video Research is a Nielsen-like company that does a top ten of the week in all genres (anime, dorama, etc.). And this site brings together figures for more than the top ten anime and posts them a few weeks later. Siegel Clyne's posts are the best early word on anime below the top ten. Thanks to bluemist for mentioning Video Research in a comment.