"I searched thoroughly and found that the best duaa` is: asking Allah the assistance to please him. And the sincere hearts and the good prayers are the never beaten soldiers." (Ibn Taymiyah) 

"O Allah, let the crescent loom above us in safety, faith, peace and Islam, and in agreement with all what You love . Our Lord and your Lord is Allah" ( Duaa` of moon sighting).

The Prophet (peace and blessings be upon him) said: "Whosoever fasts in Ramadan with faith and seeking Allah’s reward, all his past sins will be forgiven."(Al-Bukhari)

The Prophet (peace and blessings be upon him) said: "When the month of Ramadan starts, the gates of the heaven are opened and the gates of Hell are closed and the devils are chained." [Bukhari]

Allah the Almighty says:

[O you who believe! Fasting is prescribed to you as it was prescribed to those before you, that you may become righteous (achieve Taqwa) ] (Al-Baqarah 2: 183)