challenging

Adjective

• disturbingly provocative
  • an intriguing smile
    

• stimulating interest or thought
  • a challenging hypothesis
    
  • a thought-provoking book
    

• requiring full use of your abilities or resources
  • ambitious schedule
    
  • performed the most challenging task without a mistake
    

Related words

• Synonyms: 
  intriguing , thought-provoking , ambitious