The last soorah of the Qur’aan to be revealed was Soorat al-Nasr (“When there comes the Help of Allaah (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 ‘Abbaas (may Allaah be pleased with him). Muslim (3024) narrated that ‘Ubayd-Allaah ibn ‘Abd-Allaah ibn ‘Utbah said: “Ibn ‘Abbaas said to me, ‘Do you know the last soorah of the Qur’aan that was revealed in full?’ I said, ‘Yes, “When there comes the Help of Allaah (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 saheeh report from ‘Umar and Ibn ‘Abbaas (may Allaah be pleased with them both) that this soorah was a sign that the death of the Prophet (peace and blessings of Allaah be upon him) was approaching. 

Al-Bukhaari (4970) narrated that Ibn ‘Abbaas 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 Allaah says (interpretation of the meaning):  

“When there comes the Help of Allaah (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 Allaah and ask for His forgiveness when Allaah'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 Allaah (peace and blessings of Allaah be upon him) which Allah informed him of. Allaah said (interpretation of the meaning):  

“When there comes the Help of Allaah (to you, O Muhammad against your enemies) and the Conquest (of Makkah).

And you see that the people enter Allaah’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-Baraa’ ibn ‘Aazib (may Allaah be pleased with him) said that the last soorah to be revealed was Baraa’ah (al-Tawbah). 

Al-Bukhaari (4329) and Muslim (1618) narrated that al-Baraa’ ibn ‘Aazib said: “The last soorah to be revealed was Baraa’ah, and the last verse to be revealed was (interpretation of the meaning): 

‘They ask you for a legal verdict. Say: “Allaah directs (thus) about Al‑Kalaalah (those who leave neither descendants nor ascendants as heirs)’

[al-Nisa’ 4:176]

‘Aa’ishah (may Allaah be pleased with her) said: “The last soorah to be revealed was al-Maa’idah.” 

Ahmad (25588) narrated that Jubayr ibn Nufayr said: “I entered upon ‘Aa’ishah and she said, ‘Do you read Soorat al-Maa'idah?’ I said, ‘Yes.’ She said, ‘It is the last soorah to be revealed, so whatever you find in it of halaal, accept it as halaal, and whatever you find in it of haraam, accept it as haraam.’ I asked her about the attitude of the Messenger of Allaah (peace and blessings of Allaah be upon him), and she said, ‘The Qur’aan.’” This hadeeth was classed as saheeh by Shu’ayb al-Arna’oot in Tahqeeq al-Musannad. 

There is no proven hadeeth from the Prophet (peace and blessings of Allaah be upon him) which states which was the last soorah of the Qur’aan to be revealed. This is why the Sahaabah differed and each of them said what his ijtihaad led him to. 

Al-Bayhaqi (may Allaah 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-Mubaarakfoori in Tuhfat al-Ahwadhi. 

And Allaah knows best.