Apparently not many portraits of Proust were made, but those that were, were created by the cream of the late 19th century artistic crop. I was made aware of Proust portraits by an article by Oliver Trostmann in this catalog for a recent exhibit at the Isabella Stewart Gardner Museum in Boston.
Édouard Manet and (below) a painting from the same year.
Auguste Rodin made this etching of Proust.
Anders Zorn painted this portrait of Proust in 1888.
Who did the best job? Not Manet, I'm afraid. And Rodin's etching is more a technical study than a portrait showing character. So in my opinion, Zorn wins, even though his painting was one of the first portraits he made using oil paints. (In his early career, Zorn usually worked in water-based media.)