The best supplication offered on the day of Arafaah is that of the Prophet Muhammad (Peace Be Upon Him): 

"Laa ilaaha illAllaahu wahdahu laa sharika lah, lahul-mulku walahul-hamdu wahuwa ‘ala kulli shay-in qadir"

(There is no true god worthy of being worshiped except Allaah, alone, without any partners, to Him belongs the Kingdom, and all-praise is due to Him, and He is able to do all things).