Leaving group ability affects the reactivity. The electrophilic character of acetamide is minimum due to +R effect. +R effect is weaker in case of methyl acetate. +R effect is still weaker, acyl group is more electrophilic. Weakest +R group, maximum electrophilicity of acyl group. Thus the order is acetamide < methyl acetate < acetic anhydride < acetyl chloride