Dante Alighieri — the creator of the written Italian language as we know it. It might not have been too far of a stretch because had Dante written The Divine Comedy in any other vernacular, the rest of Italy might be reading, writing, and speaking that dialect.
We visited the church where his wife lies, la chiesa of Santa Margherita dei Cerchi which also happens to be where Dante met his wife, Gemma Donati and got married. Nearby is the museum where you can see some of the illustrations for his books.
In his later years, the Black Guelphs, a division of the Guelphs in opposition of the White Guelphs exiled Dante to the city of Ravenna. A tragic end for a great poet, writer, and genius.