Without a doubt, this http://www.amazon.com/Conditioner-Marpac-formerly-Sleepmate-980A/dp/B000KUHFGM
I'm not aware of any fan that would wall mount without leaving a mark. How would it attach? I looked for outlet mounted or socket mounted fans with no luck.
Here's a potential alternative (not a fan): https://www.amazon.com/dp/B000KUHFGM/ref=cm_sw_r_cp_apa_6NCBAb94PMF5B