To determine the hybridization of a complex, we need to consider the geometry of the complex and the number of electron pairs around the central atom.
- In the complex \([NiCl_4]^{2-}\), nickel is in the +2 oxidation state, and there are four chloride ions surrounding it. This suggests that the geometry of the complex is tetrahedral, and to form a tetrahedral structure, nickel will use sp\textsuperscript{3} hybridization.
- For \([Ni(CO)_4]\), nickel in the zero oxidation state has a square planar geometry, which indicates sp\textsuperscript{2} hybridization.
- \(SF_6\) has a central sulfur atom surrounded by six fluorine atoms, indicating sp\textsuperscript{3}d\textsuperscript{2} hybridization.
- \([Ni(CN)_4]^{2-}\) also has a square planar geometry, which corresponds to sp\textsuperscript{2} hybridization.
Therefore, the correct answer is (1) \([NiCl_4]^{2-}\) because it has sp\textsuperscript{3} hybridization.