For the SNAr reaction of 1-X-2,4-dinitrobenzene with sodium methoxide in methanol under identical conditions, the most reactive compound is the one with X $=$

In the addition-elimination mechanism the slow step is the attack of the nucleophile to form the Meisenheimer complex, not the loss of $\mathrm{X}{}^{-}$. The strongly electronegative fluorine makes the ipso carbon most electron-poor and stabilises the anionic intermediate, so the order is $\mathrm{F}>\mathrm{Cl}\approx \mathrm{Br}>\mathrm{I}$ (the element effect).