Berühmtste
Berühmtste is the nonstandard form sometimes encountered as a superlative of the German adjective berühmt, meaning most famous. In contemporary standard German, the attributive superlative is usually berühmste (as in der berühmteste Künstler der Epoche) and the predicative/adverbial superlative is am berühmtesten (das Werk ist am berühmtesten geworden). The form berühmtste is generally regarded as nonstandard or archaic and may appear only in older texts, dialectal writing, or as a spelling mistake in modern texts.
Etymology and morphology: berühmt comes from the stem relating to fame, with the usual German superlative suffixes
Usage and variation: The preferred form in formal writing is berühmste in attributive positions and am berühmtesten
See also: Berühmtheit, fame and recognition; other superlatives formed from adjectives describing notoriety or prominence.
In sum, berühmtste is not the normative modern form; it is generally treated as a nonstandard variant,