1979: Which is the outermost planet from the sun?
Before the discovery of Pluto in February 1930, it was Neptune. From then until February 1979, it was Pluto. For the next twenty years after that, until February 1999, Pluto’s eccentric orbit took it closer to the sun than Neptune.
Pluto should have then held on firmly to the title for the next 228 years, except that in 2006 the International Astronomical Union officially demoted it from the ranks of the planets.
So the answer depends on exactly when you ask the question. Over the last century, the answer has been: Neptune; Pluto; then Neptune again; then Pluto, briefly; and then, finally, Neptune.
Source: Duncan Brewer, The Outer Planets: Uranus, Neptune and Pluto (1992), pp. 27, 52–3