In no particular order:
Love Hina by Akamatsu Ken: fun, interesting, cute girls, and a very clean and attractive art style. Oh, and the man is the god of layout as far as I'm concerned.
One Piece by Eniichirou Oda: a simple, but grand and moving tale. It stirs up such basic emotions such as anger, excitement, and joy, but on a scale most other manga can't even touch. And the characters are all so cool. Hands down my favorite traditionally shounen series.
Great Teacher Onizuka by Fujisawa Tohru: Onizuka Eikichi is probably the most inspirational character I've had the pleasure of knowing. Not only that, he's funny as hell. A wonderful character-driven series (I happen to like those).
Card Captor Sakura by CLAMP: Sakura is one of my favorite characters of all time and Tomoyo is really up there too. While the plot wasn't much of a surprise, the characters are what make this series shine.
Video Girl Ai by Katsura Masakazu: if you don't cry while reading this, you're friggin' emotionless. One of the most moving series I've ever read.
X by CLAMP: Again, characters make the series. Two of my all-time favorite male characters are in this series: Subaru and Sorata. Besides them though, every other character is very well fleshed out. The story is overall a bit bleak, which generally isn't my cup of tea, but it's very compelling anyway. Besides, it's the pinacle of CLAMP art. ^_^
Hikaru no Go by Hotta Yumi and Obata Takeshi: sure, it's a series about Go, but it's so exciting. Very addicting.
I suppose that's all I'll list.