challenging Synonyms

ambitious intriguing thought-provoking onerous rejoining resisting rough scuffling severe significative skeptical steep stimulant stimulating stimulative recuse laborious opposed opposing problematic problematical protesting provocative question questioning rebelling rebellious mettled rebuffing subversive gruelling duelling antiauthoritarian labor-intensive brain surgery long-range call into question newsworthy hurdlesome problematically recusal repressing grueling call out taxing testing tight toilsome tough tricky