The Mona Lisa by Leonardo da Vinci.
The model, they say, was Lisa del Giocondo (aka Lisa Gherardini), the wife of Florentine merchant Francesco del Giocondo. She has been a top suspect for quite a while, in large part because the painting is known in Italy as La Gioconda. (The Louvre’s website for the painting already calls it Portrait of Lisa Gherardini, though it says the exact identity of the sitter is “unclear.”)
The Heidelberg proof is reportedly in the form of notes scribbled in an unrelated manuscript by a chum of Leonardo da Vinci in 1503, commenting that da Vinci was working on a painting of Mrs. Giocondo.