[Seig]

This hasn't been suggested yet, but you have to watch this series it's a mind fk all the way through. Higurashi no naku koro ni = When They Cry in english. I'd also suggest Monster, Gankutsuou, Gantz, and Hellsing Ultimate.
Higurashi no Naku Koroni Kai (TV) - Anime News Network

It's the best mystery series i've ever seen. Each season is 26 episodes and theres 4-5 episode arcs, after each arc time is reset and a new arc starts with small changes. Don't let the first episode fool you though, theres sort of a formula they lull you with a happy 1st episide for each arc and then the murders start. Most of the murders are committed by teens and theres some mafia style torture in an arc or two i've seen some people get disgusted by the series.
The format is basically like that movie Groundhogs Day one person constantly lives throughout the same summer over and over trying to make changes to prevent somthing that happens at the end of the summer. It's currently in it's 2nd season.

Originally Posted by Wiki

Higurashi no Naku Koro ni takes place during June 1983, at a fictional rural village called Hinamizawa (雛見沢, Hinamizawa?) (based on the village of Shirakawa, Gifu, a World Heritage Site), which has a population of approximately 2000. The main character, Keiichi Maebara, moves to Hinamizawa and befriends his new classmates Rena Ryugu, Mion Sonozaki, Rika Furude, and Satoko Hojo. Keiichi joins their after-school club activities, which consist mostly of card and board games (and punishment games for the loser, usually him.) Hinamizawa appears to be a normal, peaceful, rural village to Keiichi. However, the tranquility abruptly ends after the annual Watanagashi Festival, a celebration to commemorate and give thanks to the local god, Oyashiro-sama. Keiichi learns that for the past four years, one person has been murdered and another has gone missing on the day of the Watanagashi Festival. Keiichi himself soon becomes implicated into the strange events surrounding the Watanagashi Festival and Oyashiro-sama. In each story arc, he or one of his friends become paranoid, and a crime is committed. Usually, the crime involves the murder of one of their own friends. While it seems impossible to tell their delusions apart from the mystery of Hinamizawa, slowly the truth is revealed.