I saw this was recommended before, but with no description or review and it wasn't on the list, so here we go.
I recommend Claymore. I'm not sure how many people have heard of it, I've only talked to a few that have actually watched it, most have just heard the name. Not a very popular anime, it will never see season 2, but what there is is still great (excluding the last few episodes). The story is about an organization compromised of women who are sent out to kill youma, which are demons that are able to disguise themselves as humans they have killed. The women of the organization, nicknamed Claymores by humans because of their obvious weapon of choice, are half human, half youma and are able to sense yoki, which is the aura the youma give off, giving them the ability to sense a youma if they are disguised. The story starts out rather slow as the basic plot is pretty unoriginal, but around the 5th or 6th episode, the plot picks up and is hard to resist from then on. Now, I'm definitely not a huge fan of anime (only enjoy Kenshin/Samurai X, Yu Yu Hakasho, Berserk and I did like Naruto before it came to the US and became saturated), but this has become my favorite anime/manga of all time. The only problem with the anime is that by the end, it completely strays away from the manga and kills the reason of the series. They won't be able to continue it unless they redo the last couple of episodes. Other than that, great art style, great music, great story and only one annoying character (who becomes pretty awesome in the manga, if they could have only taken the anime that far).
The manga is written by Norihiro Yagi and the anime is directed by Hiroyuki Tanaka. The anime still hasn't been released in the US yet and was recently bought by FUNimation and is being released in October. The manga is available in most stores and scans are also available free through
http://www.onemanga.com