This problem is similar to problem 536. The only difference among these compound is their leaving group, so the compound with the best leaving group will undergo substitution reactions most rapidly.

Good leaving groups are stable. Larger ions tend to be more stable than smaller atoms (due to a smaller charge:size ratio. See problem 288), so when going down the periodic table, stability increases. I^{-} is more stable than Br^{-}, which is more stable than Cl^{-}, etc. So I^{-} is the best leaving group, and 2-iodobutane will react the fastest with a nucleophile.