 If you liked
Monster
|
 ...then you might like
Blood+
|
They are both thrillers. There's investigative action in both of them. They both have a handsome blond bad guy with a gentle voice. The bad guys in both animes are very wealthy. Both animes have monsters. They are both very addictive and the stories are very different, but they are both really great animes.
Both are slow paced anime that share the same atmosphere where the mystery is unveiled slowly, and there are some in-between yet relevant drama episodes serving as a windows for character development. Also, it is a bonus for you if you prefer an anime with a large cast of characters.