The case with which a nucleophile attacks the carbonyl group depends upon the electron deficiency, i.e. the magnitude of positive charge on carbonyl group carbon. Since, among all the groups attached (−OCH3,−Cl,−H,−O−
O
∥
C
−CH3)−Cl has maximum electron withdrawing effect, electron deficiency will be maximum at carbonyl carbon of acetyl chloride. Hence, acetyl chloride, i.e. CH3−
O
∥
C
−Cl is most reactive to nucleophilic attack at carbonyl group.