To say the least, It is one of the best anime/manga. Not just in my opinion, but that of the non-avid watchers of anime agree to this.
The story follows that Light Yagami, a good-looking teen genius discovers a mysterious notebook called the "Death Note", which belonged to the Shinigami Ryuk. It grants the user the supernatural ability to kill anyone whose name is written in its pages. While Light kills criminals to create his own version of New World, another teen genius named L, tries his best to stop him.
The plot, character development, and music are all awesome. The anime also forces you to question your own morale and thoughts about life and death. If you are into mystery or crime or detective genre, this is a must-watch anime.