I would suggest either the overall_footer or the overall_header - both in the style/templates folders.
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It is not a good idea to put Adsense code in the overall footer/header because it violates Google's terms of service. You can't have advertisements displaying on confirmation pages, for example after you post a topic. You should add the code in places such as index_body, forum_body, topic_body, etc.
There was a hack to include Adsense in the Footer, on all pages except those excluded by Google's TOS on phpBB2. Does anyone know if there is something that could be done for phpBB3?