According to Marx himself, writing later, "the salient points of our [his and Engels' new, political-communist] conception were first outlined in a scientific, although polemical, form in my 'Poverty of Philosophy'," written in winter 1846-7.
"Poverty of Philosophy" is the first text in which Marx sets out his mature outlook. It is the text in which he settles accounts with Hegel and the whole school of thought following Hegel. And it is the text in which he outlines his working-class and democratic version of communism.