The prophet peace be upon him said:

"When the fasting person breaks his fast, his supplication is not turned back."

[Sunnan Ibn Majah]

When man hears Azan (call for prayer) of Maghrib and breaks his Siyaam (fasting) by eating dates and drinking some water, he should make Du'a to Allah, Exalted and Sublime be He, because at this particular moment his Du'a is definitely answered. Ask Allah the Almighty to grant you the goodness of the worldly life and the Hereafter. 

Some of the inclusive and comprehensive Du'as are the following: 

"O Allah guide me and help me guide other people." 

Ask Allah the Almighty to guide you first and then ask Him to help you guide others. 

"O Allah, provide me with goodness and use me in good deeds. O Allah, You are the Most Forgiving and Generous, and You love forgiveness; so forgive me, O Most Generous. O Allah, make my soul obedient and purify it, for You are the best One to purify it, You are its Guardian and Lord." 

"O Allah, forgive my sins, accept my repentance, forgive my slips, help me avoid sins, and manage my affairs."

Man should learn a lot of Du'as by heart, because Du'a is considered half of worship. Better yet, worship is but Du'a. Hence, take the advice of Allah's Messenger, peace be upon him, and make Du'a to Allah at the time of Iftar (breaking one's fasting) since it is never turned back.