Outrageous

Wild

What was the most senseless thing you’ve ever done? What was the wildest or most foolhardy thing you’ve ever done? What was the most impulsive or most irresponsible thing you’ve ever done? What was the thing you’ve done that you regretted the most later?

Each of those means ‘crazy’ but the meaning is more precise, less ambiguous.

i think this might sound better

“eccentric” is close. try that.

thanks for excusing the lower-case sentences. crazy.

To be extremely formal, how about

What is the most unorthodox behavior you have exhibited?

Using ‘ever’ would be redundant to ‘most’. It is also redundant to the ‘est’ suffix.

irrational

unorthodox

unreasonable

ununderstandable

strange

odd

peculiar

singular (..it is old-fashioned)

curious

eccentric

funny

surprising

puzzling