Which ion produces a small crystal field splitting (a weak ligand field)?

Strong ligand produces more crystal field splitting. Weak ligand produces small crystal field splitting, thereby increasing strength of ligands

${\mathrm{I}}^{-}<{\mathrm{Br}}^{-}<{\mathrm{S}}^{2-}<{\mathrm{SCN}}^{-}<{\mathrm{Cl}}^{-}<{\mathrm{N}}^{3-}<{\mathrm{F}}^{-}$

Weak field ligand

$\begin{array}{r}<{\mathrm{OH}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_4^{-2}<{\mathrm{H}}_2\mathrm{O}<\mathrm{NCS}\\ ~{\mathrm{H}}^{-}<{\mathrm{NH}}_3<\mathrm{en}={\mathrm{CO}}_3^{2-}<{\mathrm{NO}}_2^{-}<{\mathrm{CN}}^{-}<{\mathrm{CO}}^{-}\end{array}$

increasing order of strong field ligands