The order of these species in electro-chemical series, from top to bottom is $ L{{i}^{+}},N{{a}^{+}},Z{{n}^{2+}} $ and $ C{{u}^{2+}} $ . In electrochemical series, as we move from top to bottom, the oxidising power increases, hence, $ C{{u}^{2+}} $ is the strongest oxidising agent among these.