Nucleophiles are the species that are rich in electrons and are capable of donating them to an electron-deficient center. The reactivity of a nucleophile depends on its ability to donate electrons, which in turn depends on factors like the presence of electron-donating or withdrawing groups, steric hindrance, etc.

Among the given options, CH3O- is the most reactive nucleophile. Here's why:

Effect of electronic factors:
- CH3O- is an alkoxide ion, which has a negative charge on the oxygen atom. This makes it a strong nucleophile because the negative charge can be easily donated to an electron-deficient center.
- C6H5O- is a phenoxide ion, which also has a negative charge on the oxygen atom. However, the presence of the phenyl group (C6H5) makes it less reactive than CH3O- because the phenyl group is electron-withdrawing and reduces the electron density on the oxygen atom.
- (CH3)2CHO and (CH3)3CO- are both alkoxide ions with alkyl groups attached to the oxygen atom. However, the presence of the alkyl groups causes steric hindrance, which reduces the reactivity of the nucleophile. (CH3)3CO- is less hindered than (CH3)2CHO and hence more reactive, but still less reactive than CH3O-.

Effect of steric factors:
- As mentioned earlier, steric hindrance reduces the reactivity of a nucleophile. CH3O- has the smallest size among the given options and hence experiences the least steric hindrance. This makes it the most reactive nucleophile.

Therefore, CH3O- is the most reactive nucleophile among the given options.