Short answer
The biggest sin in Islam is shirk: worshipping anything besides Allah or giving any creation a share of what belongs to Him. It is the one sin Allah does not forgive if a person dies upon it (4:48). Even shirk is forgiven if a person sincerely repents and accepts Islam before death.
Shirk and kufr are the gravest sins and cancel all deeds if a person dies upon them. Below them come the major sins (kaba'ir), such as unjust killing, zina, disobeying parents and consuming riba, which do not take a Muslim out of Islam but require repentance.
The four schools agree on this.
Explanation
Abdullah ibn Mas'ud asked the Prophet ﷺ which sin is greatest in Allah's sight. He said: "That you set up a rival to Allah when He alone created you." Then killing your child for fear it will share your food, then committing zina with your neighbour's wife. Luqman told his son the same thing: shirk is a great injustice (31:13).
Why is shirk the worst? Because it denies the very purpose of creation, which is to worship Allah alone. It gives to something created, an idol, a person, a saint or a force, what belongs only to the Creator: worship, ultimate trust and the power to create and control.
The Quran says Allah does not forgive association with Him, but forgives what is less than that for whom He wills (4:48). Scholars explain this verse is about a person who dies upon shirk. Another verse says Allah forgives all sins (39:53), which covers anyone who repents in this life, including from shirk. Many Companions had worshipped idols before Islam and became the best of this ummah.
After shirk come the major sins. The Prophet ﷺ named seven that destroy a person: shirk, magic, unjust killing, consuming riba (interest), consuming an orphan's wealth, fleeing the battlefield, and slandering chaste believing women. Scholars define a major sin as one that carries a fixed punishment, a curse, or a threat of Hell or Allah's anger. For the hope side of this topic, read Does Allah forgive all sins? and our guide to tawheed, shirk and kufr.
The evidence
Quran, Surah An-Nisaa 4:48
إِنَّ ٱللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِۦ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُ ۚ وَمَن يُشْرِكْ بِٱللَّهِ فَقَدِ ٱفْتَرَىٰٓ إِثْمًا عَظِيمًا ﴿٤٨﴾
Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly fabricated a tremendous sin.
Shirk is not forgiven for the one who dies upon it, while lesser sins may be.
Quran, Surah Luqman 31:13
وَإِذْ قَالَ لُقْمَٰنُ لِٱبْنِهِۦ وَهُوَ يَعِظُهُۥ يَٰبُنَىَّ لَا تُشْرِكْ بِٱللَّهِ ۖ إِنَّ ٱلشِّرْكَ لَظُلْمٌ عَظِيمٌۭ ﴿١٣﴾
And [mention, O Muhammad], when Luqman said to his son while he was instructing him, "O my son, do not associate [anything] with Allah. Indeed, association [with him] is great injustice."
Calls shirk a great injustice.
Quran, Surah Az-Zumar 39:53
۞ قُلْ يَٰعِبَادِىَ ٱلَّذِينَ أَسْرَفُوا۟ عَلَىٰٓ أَنفُسِهِمْ لَا تَقْنَطُوا۟ مِن رَّحْمَةِ ٱللَّهِ ۚ إِنَّ ٱللَّهَ يَغْفِرُ ٱلذُّنُوبَ جَمِيعًا ۚ إِنَّهُۥ هُوَ ٱلْغَفُورُ ٱلرَّحِيمُ ﴿٥٣﴾
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."
All sins, shirk included, are forgiven for those who repent.
Sahih al-Bukhari 4477
Sahih... أَنْ تَجْعَلَ لِلَّهِ نِدًّا وَهْوَ خَلَقَكَ ...
"That you set up a rival to Allah while He created you."
The Prophet's answer when asked which sin is greatest.
Sahih al-Bukhari 2766
Sahih... اجْتَنِبُوا السَّبْعَ الْمُوبِقَاتِ ...
"Avoid the seven destroying sins."
Lists the major sins, starting with shirk and magic.
Where the ruling is recorded
- al-Aqida al-Tahawiyya, Section on major sins and the people of the qibla. Explains that major sinners are under Allah's will, while shirk is not forgiven for one who dies upon it.
- al-Zawajir an Iqtiraf al-Kaba'ir (Ibn Hajar al-Haytami), Introduction on the definition of a major sin. Gathers the definitions scholars give of a major sin.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.