Synonyms of 'challenging'

alluring, arrogant, audacious, bold, brash, brassy, brazen, bumptious, cheeky, cocky, contemptuous, daring, defiant, derisive, disdainful, disregardful, exciting, galvanic, impertinent, impudent, insolent, inviting, PERT, piquant, provocative, rousing, saucy, stimulant, stimulative, stirring