The general order of reactivity of carbonyl compounds for nucleophilic addition reactions is 

The size of the alkyl group, causes hindrance to attacking group. As the number and size of the alkyl groups increase the hindrance to the attack of nucleophile also increases. Thus the reactivity follows the order

${\mathrm{H}}_2\mathrm{C}=\mathrm{O}>\mathrm{RCHO}>\mathrm{ArCHO}>{\mathrm{R}}_2\mathrm{C}=\mathrm{O}>{\mathrm{Ar}}_2\mathrm{C}=\mathrm{O}.$