In adverbial position (i.e., modifying a verb), the superlative is substantivized (i.e., made into a noun) and follows the preposition up ~ op and the neuter adjective dat ~ 't; e.g.,

Se singt up't best(e). 'She sings best.'

Under the influence of German, many Low Saxon speakers in Germany now use the preposition an instead; e.g.,

Se singt an't best(e). 'She sings best.'

Cf. German: Sie singt am besten. 'She sings best.'