It depends what you mean by 'unusual'. If you mean the eccentricity of the orbit, ie how 'stretched' it is from a circle. Then that would be MERCURY at 0.21.
However, it has only held this title since PLUTO was demoted from its planetary status, as Pluto's eccentricity was close to 0.25. Further, its orbit sometimes brought it closer to the sun than Neptune. Pluto was even more of an oddity as its orbit was inclined at 17 degrees from those of the majority of planets.
VENUS has a claim to the title also in that it is the only planet to orbit the Sun in a clockwise direction.
Take your pick !
However, it has only held this title since PLUTO was demoted from its planetary status, as Pluto's eccentricity was close to 0.25. Further, its orbit sometimes brought it closer to the sun than Neptune. Pluto was even more of an oddity as its orbit was inclined at 17 degrees from those of the majority of planets.
VENUS has a claim to the title also in that it is the only planet to orbit the Sun in a clockwise direction.
Take your pick !
