What Is the Last Surah Revealed in the Quran?
Question

What is the last Surah of the Quran to be revealed?
Praise be to Allah.The last Surah of the Quran to be revealed was Surat al-Nasr (“When there comes the Help of Allah (to you, O Muhammad against your enemies) and the Conquest (of Makkah)” [al-Nasr 110:1 – interpretation of the meaning]). This is the view of Ibn `Abbas (may Allah be pleased with him).
Muslim (3024) narrated that ‘Ubayd-Allah ibn ‘Abd-Allah ibn ‘Utbah said: “Ibn `Abbas said to me, ‘Do you know the last Surah of the Quran that was revealed in full?’ I said, ‘Yes, “When there comes the Help of Allah (to you, O Muhammad against your enemies) and the Conquest (of Makkah)” [al-Nasr 110:1 – interpretation of the meaning]].’ He said, ‘You have spoken the truth.’” 
This is supported by the sahih report from ‘Umar and Ibn `Abbas (may Allah be pleased with them both) that this Surah was a sign that the death of the Prophet (peace and blessings of Allah be upon him) was approaching. 
Al-Bukhari (4970) narrated that Ibn `Abbas said: “‘'Umar used to make me sit with the elderly men who had fought in the Battle of Badr. Some of them felt it (did not like that) and said to 'Umar ‘Why do you bring in this boy to sit with us when we have sons like him?’ 'Umar said, ‘Because of what you know of his position (i.e. his religious knowledge.)’ One day 'Umar called me and made me sit in the gathering of those people; and I think that he called me just to show them (my religious knowledge). 'Umar then asked them (in my presence). ‘What do you say about the interpretation of the verse in which Allah says (interpretation of the meaning):  
“When there comes the Help of Allah (to you, O Muhammad against your enemies) and the Conquest (of Makkah).” [Al-Nasr 110:1]?’ 
Some of them said, ‘We are ordered to praise Allah and ask for His forgiveness when Allah's Help comes to us and we prevail (over our enemies).’ Some others kept quiet and did not say anything. On that, 'Umar asked me, ‘Do you say the same, O Ibn 'Abbas?’ I said, ‘No.’ He said, ‘What do you say then?’ I said, ‘That is the sign of the death of the Messenger of Allah (peace and blessings of Allah be upon him) which Allah informed him of. Allah said (interpretation of the meaning):  
“When there comes the Help of Allah (to you, O Muhammad against your enemies) and the Conquest (of Makkah).
And you see that the people enter Allah’s religion (Islam) in crowds.
So glorify the Praises of your Lord, and ask His forgiveness. Verily, He is the One Who accepts the repentance and Who forgives” [Al-Nasr 110:1-3]
At that 'Umar said, ‘I do not know anything about it other than what you have said.’” 
Al-Bara ibn ‘Azib (may Allah be pleased with him) said that the last Surah to be revealed was Bara-ah (al-Tawbah). 
Al-Bukhari (4329) and Muslim (1618) narrated that al-Bara ibn ‘Azib said: “The last Surah to be revealed was Bara-ah, and the last verse to be revealed was (interpretation of the meaning): 
‘They ask you for a legal verdict. Say: “Allah directs (thus) about Al-Kalalah (those who leave neither descendants nor ascendants as heirs)’ [Al-Nisa 4:176]
`Aishah (may Allah be pleased with her) said: “The last Surah to be revealed was al-Maidah.” 
Ahmad (25588) narrated that Jubayr ibn Nufayr said: “I entered upon ‘Aishah and she said, ‘Do you read Surat al-Maidah?’ I said, ‘Yes.’ She said, ‘It is the last Surah to be revealed, so whatever you find in it of halal, accept it as halal, and whatever you find in it of haram, accept it as haram.’ I asked her about the attitude of the Messenger of Allah (peace and blessings of Allah be upon him), and she said, ‘The Quran .’” This hadith was classed as sahih by Shu’ayb al-Arnaut in Tahqeeq al-Musnad. 
There is no proven hadith from the Prophet (peace and blessings of Allah be upon him) which states which was the last Surah of the Quran to be revealed. This is why the Companions differed and each of them said what his ijtihad led him to. 
Al-Bayhaqi (may Allah have mercy on him) said: The differing opinions may be reconciled by noting that each of them replied according to the knowledge that he had. This was quoted from him by al-Mubarakfuri in Tuhfat al-Ahwadhi. 
And Allah knows best.