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.
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.