https://www.youtube.com/watch?v=U1YHr_CUiJ8
HunterxHunter seems like a childish shounen on the surface, and perhaps it is childish; however, this anime gets engrossingly dark on several occasions. This anime is split into 5 arcs: Hunter Exam Arc, Zoldyck Arc, Centennial Tower Arc, Phantom Troupe Arc, Greed Island Arc, Chimera Ent Arc, and the Election Arc. For those who question my claim that this show is dark, research 'phagosynthesis,' the ability to aqcuire traits from an eaten animal to pass to the offspring - something you'll see in the 'Chimera' Ant Arc. Yoshihiro Togashi, the author of the manga, also created Yu Yu Hakusho.
https://www.youtube.com/watch?v=d73yZRCx2NI
Btooom! is part of Madhous's collection of unfinished, yet magnificent, works that haven't recieved a followup: No game No Life, Btooom, Sunday Without God, and much, much more. This anime has made it into my Top 5 - up there with ShinSekai Yori: From the New World, HunterxHunter, and Fate/Zero.
https://www.youtube.com/watch?v=oWcI5mXUPWU
Witch Blade is... alright. It's something I could expect on a 2 o'clock time slot. Adult Swim should've licensed this already, even so, licensing new shows for broadcast is expensive... apparently too expensive.
https://www.youtube.com/watch?v=PkXw1iBgzoY
Death Note, a critically acclaimed psychological thriller, is said to be as intellectual as classic novels; although I digress, Death Note is a must watch. Many people know this anime from different 'Death Note' hate crimes: a kid brings a notebook to school and write other kids' names in the books; a teacher finds out and they get expelled. Rinse and repeat.