Let's take my example to give you an idea of what I mean, but by no means do you have to constrain your recommendations to my specific case.
I've read thousands of manga, prefer manga/source material over anime in general, especially in the case of adaptations. However there are some anime (adaptation or not, but most likely not) that for sure make the most of the medium. Things that a mangafag would really regret not watching on his deathbed had he only known how good they were.
Examples that come to my mind are Ping Pong (manga adaptation), Tatami Galaxy (novel adaptation), and as for original anime... nothing is coming to mind to be honest? Evangelion or Madoka I guess. Not a big fan though.
Anyway, onegaishimasu