Considering both steric and electronic effects on the addition of a nucleophile such as $\mathrm{HCN}$ (cyanohydrin formation), the order of decreasing carbonyl reactivity is

Each alkyl group reduces the electrophilicity of carbonyl carbon (+I effect) and raises steric crowding in the sp3 tetrahedral product. Formaldehyde (no alkyl groups) is most reactive, then acetaldehyde (one methyl), then acetone (two methyls); di-tert-butyl ketone is extremely hindered by the two tert-butyl groups and is least reactive.