Which halide gives a white precipitate fastest with alcoholic $\mathrm{AgNO}{}_3$ at room temperature?

Ionisation of $\mathrm{C}-\mathrm{Cl}$ is needed. Benzyl chloride forms a resonance-stabilised benzyl carbocation easily, so $\mathrm{AgCl}$ appears at once. Chlorobenzene and chloroethene have partial double-bond character in $\mathrm{C}-\mathrm{Cl}$ and do not react, and 1-chlorobutane reacts only on warming.