Nucleophilic substitution reaction depends on learning group ability
The correct order of reactivity is
$\text{RI}\text{>}\text{RBr}\text{>}\text{RCl}\text{>}\text{RF }$
It is due to fact, that weaker the base, better will be the leaving group.
Leaving group ability ${\text{I}}^{-}>{\text{Br}}^{-}>\text{Cl}>{\text{F}}^{-}$