Nucleophile always attacks on electron deficient site.
Presence of electron withdrawing groups such as $NO _{2}, CHO$ etc decreases the electron density on benzene nucleus, hence such groups activate the ring towards nucleophilic attack. While presence of electron releasing groups such as $R$ or $OR$ increases the electron density, thus deactivates the nucleus towards nucleophilic attack.
$NO _{2}$ group activates the ring more than $Cl$ towards nucleophilic attack,
