Boys over Flowers (Hana Yori dango)

Maybe the best shoujo manga ever. This story has so many ups and downs that by the time you're done with the series, you'll probably be gasping on a hospital bed after, say, five heart attacks. This story follows the life of Makino Tsukishi, and the events that follow when she starts attending the rich Eitoku Academy on a scholarship. And did I mention that she gets on the bad side of the school's most violent group - the F4? (series width - 36 volumes - complete)

Akira

You're really missing out if you've haden't read this shonen sci-fi thriller yet. This complex tale is how Kaneda - the leader of a notorious biker gang - discovers a secret government experiement that has gone so terribly, terribly wrong. (series length - 6 volumes - complete)

Uzumaki

Do you like horror manga? Well this one's perfect. It's about a city that gets possessed by a presence that dominates eveything in the shape of a spiral. It's pretty creepy, and the artist does a great job of portraying that in the story - and in his art. (series length - 3 volumes - complete)

Parasyte (kiseiju)

This story is a mixture of horror and sci-fi. These parasitic, wormlike creatures land on earth and take over living things by going inside their bodies and  entering their brains. Shinji, a middle school student, almost gets infected by one until he manages to grab the parasite before it enters his head. Instead, the parasite ends up living on his hand. The rest of the story shows how he tries to coexist with the parasite and tries to kill other parasites before they dominate other human's bodies.   (series length - 10 volumes - complete)

Berserk

This manga probably has the most blood and gore that I've seen in awhile. But the story is fast-paced, suspensful, and really entertaining. If you enjoy violence, this series is perfect for you. (series length - 31 volumes - ongoing)

If you'd like to read some of these stories before deciding to buy them, check out http://www.thespectrum.net/manga_scans/ for high-quality manga scans and a full list of great manga. Check it out!