A few to start:
Fruits Basket , brilliant drama and comedy (based on the chinese zodiac).
Seitokai no Ichizon , the life of a student council, same setting of Yakuindomo (one guy and several girls), different development.
Ergo Proxy , future setting, dystopia, wonderful animations.
Higurashi no Naku koro Ni , comedy becomes horror in a rural setting. It's important to watch the second season too because it contains answers not given in the first. The OVA set is a bonus.