For $\mathrm{Z}-\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{OCH}{}_3$ eliminating $\mathrm{CH}{}_3\mathrm{OH}$ with $\mathrm{NaOEt}$ by E1cB, the substituent Z which gives the fastest elimination (the most acidic hydrogen alpha to Z) is

The rate of E1cB follows the acidity of the C-H bond alpha to Z, i.e. the stability of the carbanion. Approximate pKa values of the corresponding carbon acids: nitroalkane 10 (nitronate), ketone 20, ester 25, and a simple phenylmethyl group about 40. So $\mathrm{NO}{}_2$ gives the fastest elimination.