um, how much money have you got? cuz you can spend a fortune if you try to visit places like Oz, Japan, or Europe for SIX MONTHS. they'd be great for 1-2 months at a time, but a 6-month-stretch will be costly.
but if money is no object.... i'd say europe. i would have said japan, but 6 months might be too long if you've never been there - language and culture differences would have you hoping for a break after a short while.
or if you're slightly bold, do the Mediterranean loop - portugal spain france italy grece montenegro turkey egypt algeria those other countries and finish in or around morocco, or carry on across gibraltar back to portugal.