I would like to know which is the preferred Dua that is said after leaving the toilet , is it "ghufranaka" or is it "al hamdulillahi llathi athhaba a'nil atha wa a'fani? Please reply soon,

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is narrated from Aisha that the Prophet (Sallallahu Alaihi wa Sallam) used to say after going out of the toilet; Ghufranak i.e "O! my Lord forgive me".
Abu Dawood also reported from Abu Dhar that the Prophet (Sallallahu Alaihi wa Sallam) used to say after leaving the toilet; "Thanks to Allah who has removed harm from me and granted me peace."
Many scholars said that Abu Dhar's Hadith is weak. But they said that Aisha's Hadith is sound. These scholars include Al-Nawawi and Al-Munziri who said: "The chain of Abu Dhar's Hadith is weak and this is why Abu Hatim Al-Razi reported that the most sound Hadith about this issue is Aisha's."
Allah knows best.