Yu Yu Hakusho
Yu Yu Hakusho (which could be roughly translated into 'The Poltergeist Report') is an excellent story consisting of 19 manga books written by Yoshihiro Togashi. The basic plotline is like so:
There are three worlds: Ningenkai, Makai, and Reikai. The Human World, Demon World, and Spirit World in that order. The Ningenkai and the Makai are seperated by a thin barrier between dimensions, which the inhabitants of the Makai, youkai, could penetrate easily. The youkai, demons, are well aware of the presence of the Ningenkai, as well of the Reikai.
The Reikai is where the souls of those from the Makai and the Ningenkai go after their death. The souls are then judged by the rulers of the Reikai and taken to their proper places on basis of their actions in life. The Rekai also act as guardian against the Makai from invading the much weaker and quite unaware Ningenkai. Actually the assigned Reikai personages move to stop any side from crossing the barrier between worlds, except the Ningens, humans, don't even have a clue, so there's no problem with them.
The Makai is *big*. Did I say BIG? There's an enormous diversity of youkai living there. Including partially sentinent plants. Carnivorous plants that have fangs and withering tentacle-vines. Very impressive. The Makai world is also very similar to the Ningenkai with its geography, but far more deadly. All youkai have some sort of minor power, but there are a few who are extremely powerful. Strong enough to blow up mountains with a single wave of their hand, using their youki. Youki is demon energy, more accurately, it's the natural energy of a demon. Some demons could work on their potential and build up their original status. All the people are ranked by classes. S-class is the most powerful, those in this class could defeat any of the lower classes. From S-class follows A-class, then B-class, C-class, D-class and so forth. People in whatever class could defeat those in the lower classes below them. So, basically the Makai is one huge battleground. It was divided into three kingdoms that were by the youkai: Yomi (one many-eared dude), Muruko (1/3 machine, sort of, lady), and Raizen (who is just behind this huge little secret…).
The story we follow in this background is of a 14 year old deliquent named Yuusuke Urameshi. He's a good kid at heart, but is one of the most feared fighters at a gang-riddled school. His sudden death bought on by a car accident was unexpected, not to mention meaningless, by the Reikai. Thus, he was given a rare second chance of resurrection on the condition of becoming a Reikai Tantei, Spirit World Detective, one who protected the Ningenkai against the Makai. He accepted and was eventually resurrected.
After his resurrection, Yuusuke became a psychic detective/troubleshooter. He received many assignments by his "boss," Koenma Daioh, Prince of the Reikai and the son of Enma Daioh. On his missions, he joined up with an old rival Kazuma Kuwabara, who was a strong psychic. Later, two demons also joined his team: Hiei, a powerful fire demon, and Shuiichi Minamino, the reincarnated youko Kurama. Both were dangerous youkai, well known and feared.
Slowly becoming close friends despite having come together by punishment, choice, or condition, the quartet faced renegade demons, power-crazy B-class-and-above beings, and other tough assignments. Around the middle of the anime series they join the Ankoku Bujutsukai, the Dark Tournament, which lasted roughly 7 books of the manga series. Then after that challenge, there was this near-fatal assignment with Sensui, the first S-class personage they run into. Sensui was a powerful psycho influenced by the Black Tape that wanted to open a permanent portal between the Makai and the Ningenkai. The Black Tape was something that could make it's watcher become hard-wired with hatred for all humanity within five *minutes* of viewing it for most people. Yuusuke died for a second time with the fight against Sensui and resurrected by himself, becoming S-class because of some certain DNA in his blood… That dangerous episode lasted for 4 books, and the remaining two books of the manga series wraps up the story for us though it appears the tale continues on with assignments for them.