challenging (adj.)

• alluring
• arrogant
• audacious
• bold
• brash
• brassy
• brazen
• bumptious
• cheeky
• cocky
• contemptuous
• daring
• defiant
• derisive
• disdainful
• encouraging
• energizing
• exciting
• galvanic
• galvanizing
• impertinent
• impudent
• insolent
• inviting
• pert
• piquant
• provocative
• provoking
• rousing
• saucy
• stimulant
• stimulating
• stimulative
• stirring