Which two neighboring planets are the furthest apart?

The two neighboring planets that are furthest apart in our solar system are Neptune and Mercury. While their average distances from the Sun are about 30.1 astronomical units (AU) for Neptune and 0.39 AU for Mercury, the vast difference in their distances means that when they are on opposite sides of the Sun, they can be nearly 30.5 AU apart. This significant separation highlights the varying distances of planets in our solar system.