I would say
Rosario + Vampire but it does involve ecchi moments... but I really recommend you to read it! Once you get into it, you will forget about echi. Besides it isn't that bad as in anime.
The drawings are not that great at first but as manga progresses, art becomes fantastic.
It also involves harem... but main pairing is kinda established from the begining.
There are a lot of different gangs and organizations that are after the main cast, some are weak but some are really strong. It has nice action too.
The main heroine Moka is very strong when her Rosario is taken off and she is also very cool.
The main hero Tsukune is weak and sometimes a bit annoying at the begining but eventually he grows up as a very cool and strong character.
I really suggest you to give it a try despite some ecchi elemnts and slightly weak art at the start.
It also has 2 parts, first part has 40 chapters and second is still on going and is actually better than the first.
Here are 2 images for comparison:
Spoiler for space:
Begining of first season:
Somewhere in the middle of second season:
