chal·leng·ing
 /ˈʧælənʤıŋ/  adj   [more ~; most ~]
  : difficult in a way that is usually interesting or enjoyable
   I find the job challenging and fun.
   Teaching is challenging but rewarding work.
   It's a challenging book that requires careful reading.
   The slope was challenging for even the most experienced climbers.
  chal·leng·ing·ly  adv