Not gonna repeat the obvious big names, so I will try recommend a few less known series:
1. Beelzebub. New series with only 8 chapters out. Action/Delinquent fighting type with a little twist. It's still too early to judge properly but so far it show potential;
2. Binbougami Ga! On going series with 3 Dankobon (3rd one coming out May 1st). The two protagnists are both girls but the series is really more shonen than shojo. It's funny, with nice developments so far. Nothing epic, but definitely good entertainment.
3. Ai kora (aka Love&College). Ongoing series by same guy who did Midori no Hibi. If your taste for RenAi manga are at To-Love-Ru level, whatever makes you happy; But if you want more than just softcore porn, try this one out. It's got some weird and funny gags AND an actually developing story. I thought this was an unknown series yet the other day I just found out it ranks 18 on onemanga's popularity list.