The least value of magnetic field strength required to magnetize is nickel because it is a ferromagnetic material. $\mathrm{Ni}$ is a ferromagnetic substance which gets strongly magnetized when placed in an external magnetic field. When external magnetic field ${\mathrm{B}}_0$, is applied the magnetic domains orient themselves in the direction of magnetic filed and grow in size.

Hence, $\mathrm{Ni}$ requires least value of magnetic strength. $\mathrm{NaCl}$ is a diamagnetic material. Its field lines are repelled.