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.