When I started to watch anime again and asked for something at the level of Death Note, Fernando recommended me Monster.
The first 4 or 5 episodes made no sense to me, but I kept on going and it turned out to be an awesome story, with a lot of side-stories that merge into the main one, and in the end, those first four episodes make total sense and are required to understand the late ones. The 74 episodes make just one storyline, not like most animes out there, and it includes a lot of characters. Most of these characters happen to die a lot, specially when they are close to finding the truth about all the murders that happened in the last 10 years.
This is not an anime suitable for children, since it includes graphic violence, prostitution, child abuse, etc… but it makes it more real and creates an awesome sense of immersion in the story. So much I watched 10 or more episodes per day.