Clannad After Story, the number of twists and turns it takes, the emotional drama, the light comedy, everything was perfectly balanced. It's former series, just "Clannad" was a harem, romance, comedy, high school series (though when I say harem I don't also mean ecchi). When After Story came out I knew it was a continuation of Nagisa and Tomoya trials of high school and life but never did I imagine it would take such emotional involving twists and turns.
You begin with the simple pairing of Nagisa and Tomoya the typical high school laughs, but the nature of Nagisa's illness holds her back and Tomoya goes on to find work. The story unfolds into something remarkable about the importance of family. It can be a little unbearably emotional in the twists near the end, but the way it is presented makes it easily one of my all time favourites for dealing with such issues with a great deal of maturity and emotion. The very last episode, changes everything all the questions are finally answered, though I can't really explain it without spoiling anything. It has by far been the most remarkable series I've ever watched.

Though if you mean mindboggling as in horror, psychological or mystery I'd say Higurashi No Naku Koro Ni or Paranoia Agent XDD.

Hmm not exactly? Hanasakeru Seishounen is a decidedly average anime I'm watching at the moment, it's like Fushiigi Yuugi (melodrama and rediculously exagerrated scenes) meets Saiunkoku Monogatari (bishounen reverse harem with guys who will do anything for...) Yuuki in Vampire Knight (annoying female lead who repeatedly doesn't listen and gets in trouble). Too much of it is boring, predictable, shallow and overdone.