Well, I have some ideas that I thoroughly enjoyed reading/am reading.
Black Butler(Ongoing series, so far its great)
Madoka magica-(Including some spin-offs/Side-stories I haven't been able to get the copy of.)
Hunter x Hunter(Ongoing series, its nicknamed hiatus x hiatus for a reason so be warned)
Goodnight PunPun(Completed, deeply psychological and contains mature themes)
Bastard(Completed) - I'm actually recommending the manhwa, but there is a manga under the same name("Bastard!!") that is an entirely different series.