The Prophet (SAW) said: The best invocation is that of the Day of Arafat, and the best that anyone can say is what I and the Prophets before me have said:
237. Laa 'ilaaha 'illallaahu wahdahu laa shareeka lahu, lahul-mulku wa lahul-hamdu wa Huwa 'alaa kulli shay'in Qadeer.
None has the right to be worshipped but Allah alone, Who has no partner. His is the dominion and His is the praise, and He is Able to do all things.
Reference: At-Tirmithi. Al-Albani graded it good in Sahih At-Tirmithi 3/184, and also Silsilatul-'Ahadith As-Sahihah 4/6.