Since I participated in the anime poll I figured I might as well not neglect the manga poll either.

1. Phoenix - Arguably the greatest piece of "sequential art" (Will Eisner's all-embracing terms for comics) ever drawn. Tezuka's ability to tell a story through art is unrivaled. A massive time-traveling (and alas, unfinished) epic about man and woman, war, life and death and love. Received an excellent TV anime adaptation.
2. Fruits Basket - I was already a committed anime/manga fan when I discovered this series, which just plain stole my heart. The anime was great but was only a fraction of the whole story.
3. Yotsuba&! - Adorable. Funny. The only manga that my mother reads (and she's addicted. Looking forward to loaning her volume 10).
4. Ranma 1/2 - Good action, good comedy, but I mostly love this series for the vivid characterization that Takahashi creates. She's also very underrated as a graphic storyteller.
5. Hayate X Blade - Come to think of it, I guess I like this series for the same reason I like Ranma 1/2. So just read that justification again, substituting the name "Hayashiya."
6. Shoulder a Coffin Kuro - A very strange work about an oddly gothic world with magic and weird science: a young girl on an obscure quest with strange companions. Hearttugging at frequent intervals. And then, bizarro 4koma humor. I'm not sure I know what's going on, but I love the series.
7. Iron Wok Jan - A hot-blooded shonen fighting manga of the "tournament" genre. Oh, and it's about cooking. Hilarious, insane, the most obnoxious protagonist you can imagine, cooking techniques and ingredients that defy belief... and you wish you could taste them. Miraculously, all 27 volumes were released in North America.
8. Lone Wolf and Cub - Ogami Itto's quest for revenge in a hellish Tokugawa-era Japan. I don't normally go for this genre, but the sheer power of this series won me over.
9. My Heavenly Hockey Club - I love most of Ai Morinaga's work and this series is my favorite. It's virtually (if not actually) a parody of Ouran High School Host Club, which is already a hilarious comedy. And yet, Morinaga's work manages to be even funnier by making each of the characters even more so. And no angst allowed.
10. Gals - Though I was very happy that the second season of Super GALS got a North American DVD release, I was happier yet about the complete manga release of the original source material, as (as above with Fruits Basket) not the entire story was adapted. A nice light romantic comedy with occasional dramatic bits and great characters.

This top 10 consists only of manga series that have received an official US translation and release. At least that's my justification for not including Umi no Misaki. It certainly can't be that I completely forgot about that series.