challenging

If something is challenging, you better bring your "A game." It isn't going to be easy but you will get to test your skills as you push yourself, like completing a challenging crossword puzzle or a challenging hike up a mountain.

Another meaning of challenging is "causing to question values or assumptions," like a challenging movie that makes you re-evaluate your attitude about something. In other words, it challenges what you believe. People sometimes use challenging as a polite — and far more positive — substitute for troublesome or problematic, as in, "This challenging situation will require everyone's patience."