How to determine the order of nucleophilicity for given chemical species?
Within a group of nucleophiles with the same atom, the nucleophilicity decrease with decreasing basicity of nucleophile.

Decreasing basicity means the decreasing affinity of electron pair for a proton. The decreasing order of nucleophilicity is shown in the figure. 



But the relationship between nucleophilicity and basicity can be reversed by steric effect. Less basic but steric unhindered nucleophile therefore have higher nucleophilicity than strong basic but strically hindered nucleophile.