My DD16 and I have done them all and, with her help, we've voted for Japan, France, and China. To stay spoiler free, Perry the Platypus does some cool action-y things in Japan and France and we loved China because one of the clues is obtained in a pretty cool way involving a code word and a kidcot representative.
But, honestly, we'd do all of them again except Mexico and that was because the last time we were there some of the Mexico effects weren't working properly.
Have fun with whatever you decide!