Mentally stimulating anime (Whiny, demanding anime request! :D)?

[Inkwolf]

I'm looking for an anime that engages my brain!

Maybe something that has a mystery that keeps you guessing.
Maybe something that makes you re-examine social and moral absolutes.
Maybe something with an incredible battle of wits.
Maybe something that examines human nature.
Maybe something that calls reality as we know it into question.
Or maybe just something so weird it's mind-bending.

Some things I've seen like this would be:
Spiral
Death Note
Eden of the East
Fantastic Children
Paranoia Agent
Kino's Journey
Noein
Princess Tutu
xxxHolic
Revolutionary Girl Utena

In short, something that puts questions into your head.


DO NOT WANT: Gore-fest, angst-fest, excessive nudity or non-stop-action. I don't mind a bit of blood, fighting or drama when called for, but I'm totally not into shocker violence or constant misery and most fighting action scenes put me to sleep. Uber-ecchi is also a bore. I'm not a teen boy, no interest at all in staring at cartoon hooters.

Please do not mention Code Geass, Elfen Lied, Higurashi, or any mass-market mainstream stuff with half a jillion episodes.

Already thinking of watching Sayonara Zetsubo Sensei, feel free to add your opinion if you've seen it.