Rashi רש"י
commentator on Tanakh and Talmud, without whom neither is studied
Rabbi Shlomo ben Yitzchak was born in Troyes in Champagne in 4800, studied in the academies of Worms and Mainz — heirs of Rabbeinu Gershom — and returned home to open his own school. Tradition has him living from a vineyard; his commentaries are full of exact observation of work and craft.
He did the hardest thing: he explained everything. His commentary on the Tanakh answers the question the reader has not yet formed, in two or three words, joining the plain sense to midrash. His Talmud commentary is a near-impossible feat: it walks the reader through a compressed Aramaic argument, explaining every turn without getting in the way. Without it the Talmud is practically unreadable.
Where Hebrew failed him he wrote in French: some two thousand Old French words survive only in his glosses, a treasure for linguists.
He had no sons, but his three daughters married scholars, and his grandsons Rashbam and Rabbeinu Tam founded the Tosafist school that argued with him on his own page.