Oh god...what to recommend...
My tastes are a bit...adult...to put it mildly. I'd recommend Battle Royal in a heartbeat, but it's so freaking graphic @_@
GTO is definitely worth a read, it's great. It's got so many funny parts that anyone who likes Cro-high will like GTO as well.
For Shojo, I recommend Nana, as it's not your typical girl-meets-bishonen Shojo manga, It feels like what's happening is real, and you start to really feel for the characters @_@
I couldn't wait for Nana 12 to be translated so I went online to find scanlations. I NEVER do that for a manga, I always buy them (it's my vice).