Can you tell me when Dua gets accepted 100% by Allah? Or in which Namaz Dua is get accepted by Allah 100%? Please tell me that.
When talking about Du'a we speak about a conversation between us and our Creator.
There's no magic formula like a certain du'a we may say so that our supplication might be automatically accepted.
One must say his supplication with all his might and mean it and believe that it will be accepted, these are a few conditions out of many that might help.
But we also should know that if we are sinning and our income is not halal we should worry about the acceptance of our supplication:
O people, Allah is Good and He therefore, accepts only that which is good. And Allah commanded the believers as He commanded the Messengers by saying: "O Messengers, eat of the good things, and do good deeds; verily I am aware of what you do" (xxiii. 51). And He said: "O those who believe, eat of the good things that We gave you" (ii. 172). He then made a mention of a person who travels widely, his hair disheveled and covered with dust. He lifts his hand towards the sky (and thus makes the supplication): "O Lord, O Lord," whereas his diet is unlawful, his drink is unlawful, and his clothes are unlawful and his nourishment is unlawful. How can then his supplication be accepted? (Sahih Muslim)
Some other recommendations: Is to do supplication on a time which is known as a time where Allah may accept it such as the hour of Jumu'ah, after the prayers, in the night etc. We should start it by praising Allah and sending prayers and blessings to our beloved Prophet () and we shouldn't forget to thank Allah during our supplication and if possible add some known du'a formulations from the Qur'an and sunnah.
There are three du'a's which are certainly accepted: that of an oppressed, that of a traveller and that of a parent for his kid:
Three supplications are answered without doubt. The supplication of the oppressed, the supplication of the traveller, and the supplication of the parent for his son." (Riadh-u Saliheen)
"Be afraid, from the curse of the oppressed as there is no screen between his invocation and Allah." (Sahih al-Bukhari)
Finally if Allah delays the acceptance or doesn't accept our supplication that might not necessarily mean that he refused, but it might also mean that he had something better for us as he know us best and knows what is the best for us:
He knows that which deceives the eyes and what the breasts conceal. (40:19)