In Genesis, Shem is named first among Noah's three sons. Genesis 5:32 and 6:10 list Noah's sons as Shem, Ham, and Japheth.
The order is a textual fact, but it does not by itself settle every question about age or chronology. Genesis 9:24 calls Ham Noah's “youngest son,” while Genesis 10 presents the descendants of the three sons in a different order, beginning with Japheth. Readers therefore should not assume that every list in Genesis follows the same purpose.
Shem is especially prominent in the later genealogical narrative. Genesis 10 traces his descendants, and Genesis 11 continues the line from Shem to Abram. This genealogy is why later biblical tradition associates Shem with the ancestors of several peoples, including the Israelites.
The answer is Shem, not Japheth, even though some passages discuss Japheth first in a different context. The question concerns the order in which Genesis names Noah's sons in its early Flood passages.