Reactivity depends on reaction. You can't talk about reactivity without having a reaction. For instance, in an acid-base reaction, I would be most reactive. For reactivity at the carbon attached to the heteroatom, I would also be the most reactive because it has the more electronegative atom attached via double bond to a carbon, rendering that carbon very electrophilic. The formation of the acid activates the carbonyl, making it more reactive by increasing the positive charge on the carbon (draw resonance form).
