Sayyidul Istighfar- The Best way of asking Allah for forgiveness:

`Allahumma Anta Rabbi, la ilaha illa Anta, khalaqtani wa ana `abduka, wa ana `ala `ahdika wa wa`dika mastata`tu, a`uzhu bika min sharri ma sana`tu, abu'u laka bini`matika `alayya, wa abu'u bizhanbi faghfir li, fa innahu la yaghfiruzh-zhunuba illa Anta.
 
O Allah, You are my Lord, there is none worthy of worship but You. You created me and I am Your slave. I keep Your covenant, and my pledge to You so far as i am able. I seek refuge in You from the evil of what I have done. I admit to Your blessings upon me, and I admit to my misdeeds. Forgive me, for there is none who may forgive sins but You.


The Prophet Muhammad  said “If somebody recites it during the day with firm faith in it and dies on the same day before the evening, he will be from the people of Paradise and if somebody recites it at night with firm faith in it and dies before the morning he will be from the people of Paradise.”

 
[sahih Al-Bukhari. Other reports are in An-Nasa'i and At-Tirmidhi ]