The correct order of leaving group ability in a nucleophilic substitution reaction is 

Weak bases are the better leaving groups . 

The basic strength are $B{r}^{-}<C{l}^{-}<C{H}_{3}CO{O}^{-}<H^{-}$

Hence the leaving ability are $B{r}^{-}>C{l}^{-}>C{H}_{3}CO{O}^{-}>H^{-}$