For me, it's World at War.
It really captures that sense that war is hell and feels like a desperate and morally ambiguous struggle for survival as opposed to the more detached shooting galleries of most entries in the series. I feel it's the most intense and truly mature entry in the series.
Black Ops II gets an honourable mention for its branching narrative paths, interesting near-future setting and technology, and the best story in the series in my opinion.