Shonen?
- Black Cat. It's pretty interesting, but I've only read a few chapters of it. :P
- One Piece. Have you read it in Shonen Jump? This is probably one of the few shonen manga I like.
- Prince of Tennis. It's a fun read. But it depends on your preferences.
Shoujo?
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Mekakushi no Kuni aka Land of the Blindfolded. Click the link to read more about it. It's a must read.
- Nosatsu Junkie. There's only two volumes out, but it's pretty cute. The average cross-dressing story, though.
- BXB Brothers. Funny and random. Get it from shoujomagic.
- Nodame Cantible. There's a thread about it somewhere on this subforum. It's really funny.
- Ouran High School Host Club. It may be full of bishounens and revolves around a boys harem, but it's so funny that guys and girls alike can enjoy the manga. The anime's awesome as well.