One of the all time greats. I must second the mention of The Languages of Pao. I would add To Live Forever, The Blue World, Emphyrio and The Gray Prince as some of his best novels. He excelled at the novella, too, giving us classics like The Houses of Iszm, The Moon Moth, The Last Castle, The Dragon Masters and the Magnus Ridolph novellas fixed up as Monsters in Orbit.
As seen above, his series were very popular, especially his fantasies. Mostly I don't find those up to the standards of his standalones. But second-tier Vance is exceeding high standard anyhow. I even like his Brains of Earth!