Some of the mangas (I'm not sure of its plural) I'd definitely recommend are the following. I will spare the details to minimize spoiling the curiosity of searching for one in a bookstore.
1. Tramps Like Us a.k.a Kimi Wa Pet- (14 Volumes) Romance; Contrary to its suggestive title, it's actually a cute not-so ordinary love story between an older woman , a mature guy and a talented boy who's her pet (above 18).
2. Suki- (3 Volumes) Another romantic weird relationship between a naive girl and her teacher. If you're familiar with the works of Clamp, this a definite must read.
3. Crescent Moon- (5 Volumes) A girl who encounters a tribe of demons. My nieces love the drawings and the graphics are detailed.
4. Dream Saga - (5 Volumes) Like Card Captor Sakura, work of art by Megumi Tachikawa too. It's about a little girl she catches a red magic stone that mysteriously falls from the sky. My nieces love this one too.
5. Confidential Confessions - (Unsure) The manga deals with many of the hard-hitting issues that teens face today. It's very brutal but it actually has good lessons to teach. This has mature content all over.
Anyway, some of these titles are difficult to find since they are quite expensive (prices range from 160 php to 500 php, National Bookstore used to sell it at 79 php then at 50% off a little later but that was just something that happened once). If you want to read them online, I suggest you go to www.mangafox.com. They have a wide selection for the manga lovers to feast their eyes on.