One of the oldies, Ranma 1/2 is the best I've read so far. It has everything that I could want in a manga. Action, romance, comedy, twists!
This story is about a boy named Ranma who falls into the lake of the drowning girl. Now every time he is hit with cold water he turns into a girl and can only change back when hit with hot water. But then he finds out that he is engaged to a girl named Akane. Both of them hate each other, are good with martial arts, and have a fiery temper. Together, and with the help of some quirky friends, they go through insane adventures.
Personally this didn't seem like it could go anywhere as a series. But as I continued reading I was surprised! Everything about this series just compelled me to read more and more and more.
This is just so amazing and I would highly suggest this to anyone of all ages!