I believe it is Venus since it is the closest planet to the sun with an atmosphere. While Mercury may be the answer, I think Venus is more probable since the atmosphere traps a tremendous amount of heat--however, I may be wrong. Although, this problem could be easily google searched.
Venus is, on average, even hotter than Mercury. The reason is because Mercury is tidally locked, so the side opposite the sun is quite cold. Venus has a dense heat-trapping atmosphere, and thus its average surface temperature is quite high.
