Step 1: Analyze the reactivity of carbonyl compounds.
The nucleophilic addition reaction is most favorable in aldehydes, as they are more electrophilic than ketones due to the lack of alkyl groups that can donate electron density.
Step 2: Conclusion.
The aldehyde \( \text{CH}_3 \text{CHO} \) is the most reactive towards nucleophilic addition.
Final Answer:
\[
\boxed{\text{CH}_3 \text{CHO}}
\]