A full list of countries that interest you would be helpful, but from those, I'd say Brazil.
There's plenty of people who need help and you can enjoy yourself on your free time. If you don't plan to have any free time and want to contribute the most to your CV, choose some country in Africa (Kenya comes to mind).
I would personally choose one of the slum towns near Rio de Janeiro in Brazil, but if you're going there, learn at least enough Portuguese to be able to communicate with those who don't know any English (or Japanese, that's their second language if I'm not mistaken) - in Africa you would most likely need a translator.