Which planet has the most eccentric orbit?

Mars has the most eccentric orbit in the solar system. Where 1 astronomical unit is the distance between Earth and the sun, the orbit of Mars varies from 1.36 to 1.64 astronomical units. This eccentricity combined with its axis tilt provide Mars with seasons that vary dramatically from one another.

Because of its orbit and the exchange of carbon dioxide between its ice caps and the atmosphere, the atmospheric pressure on Mars during winter is 25 percent lower than it is during summer. Before Pluto was downgraded to a dwarf planet, it had the most eccentric orbit, one that varied between 29.7 and 39.5 astronomical units.