It’s a pretty straightforward question so I don’t know if my answer will be in depth enough for this sub, but the answer is probably either Josquin des Prez (born c. 1450s) or more likely Giovanni Pierluigi da Palestrina (born c. 1520s.)

Bach was born in 1685, and the established Baroque style at the time of his birth and education was heavily influenced by those two Renaissance composers, among others. One of the key elements of Baroque music is polyphony, the idea of multiple complementary or competing melodies occurring simultaneously. Josquin did a great deal to popularize and establish this feature in his composition- in fact, his choral Ave Maria is supposedly the piece that convinced the Roman Catholic Church that this exotic, new-fangled polyphonic sound was actually appropriate for sacred music. Palestrina, who came much later, is widely considered to be the pinnacle of Italian Renaissance music, and had a similar polyphonic composition style that emphasized walking musical lines that did not ‘leap’ a lot of notes at a time. There are surviving copied transcriptions of some of Palestrina’s pieces handwritten by a young JS Bach as part of his studies, and the influence is apparent in Bach’s own compositions.

Not a lot is known about Josquin’s life, and knowledge of his style is complicated by his fame during his life and shortly after his death as quite a few pieces are believed to be attributed to him erroneously due to name recognition. His work was famous enough that there were records of high profile people like Martin Luther and King Louis XI praising him.

Palestrina was likely more famous than Josquin at the time of Bach’s youth and education. For a period of about ten years in the 1540s-1550s he was the choir director at St. Peter’s Basilica in Vatican City, and by that time he was already relatively well known. A widower himself, he eventually remarried a wealthy widow in his 50s which gave him the financial independence to compose and teach prolifically until his death at age 69, after which students of his continued his work and spread his style of composition, further cementing his reputation in the music world.