The du’aa that is commonly known is:

“O Allaah, for You I have fasted and by Your provision I have broken my fast” narrated by Abu Dawood, 2358. It is a mursal hadeeth so it is considered da’eef (weak). Al-Albaani classed it as da’eef in ben dawwad (510)
The one that is more authentically reported is from the hadith of Umar (may Allaah be pleased with him) who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) used to say when breaking his fast:

‘Thirst is gone, the veins are moistened and the reward is certain if Allaah wills.’ Narrated by Abu Dawood, 2357; al-Daaraqutni, 25.
Therefore the correct way of breaking the fast is to say Bismillaah when its time for Iftaar and eat a date and drink some water or juice and then make the above mentioned Du’aa narrated by Umar (may Allaah be pleased with him).
Dr Khalid Khan