Well, I don't know if anyone's mentioned it, and I've never read it, but I highly recommend the anime version of Kuroshitsuji. It's amazing.
Black Cat, Manga only, as well. However, the beginning's very different from what comes later, and depending on which you like more, it either starts off really weird or becomes really weird. It's not a quick transition, though, so it's difficult to tell. Don't watch it, though. They really messed up the character designs.
Fullmetal Alchemist, both. This is amazing. No questions, no anything. Quite near the best anything.
While I haven't seen the anime, Pet Shop of Horrors is an absolutely amazing manga. The sequel doesn't disappoint, either.
If you can find it, read Bizenghast. Although it's not officially manga, it is licensed by Tokyopop and is creepy and wonderful.
While I can't say I recommend watching Claymore, I can highly recommend reading it.
Finally, there is Negima! There are two anime versions, neither of which is very good from what I can gather, but the manga is incredibly entertaining and well-drawn. Besides, it's by Ken Akamatsu, who also did Love Hina. However, if you're going to read this one, don't read it online unless you can find a very large and thorough scan. There's a lot of small print that can be really hilarious, and that is really worth reading. It's licensed by Del Rey, and they do a pretty good job of translating.
Also, if you like Bleach, I really recommend an earlier work of his, Zombiepowder. It's only four books long, but really good.
