Abu Hurairah (May Allah be pleased with him) said: The Prophet (PBUH) said, "Keep away from the seven fatalities." It was asked: "What are they, O Messenger of Allah?" He (PBUH) replied, "Associating anything with Allah in worship (i.e., committing an act of Shirk), sorcery, killing of one whom Allah has declared inviolable without a just cause, devouring the property of an orphan, the eating of usury (Riba), fleeing from the battlefield and accusing chaste believing women, who never even think of anything touching their chastity."[Al-Bukhari and Muslim].
All the sins mentioned in the Hadith are major sins but Shirk is the greatest of all. The reason behind this is that it will never be pardoned, and one who commits it, will abide in Hell for all eternity. So far as the other major sins are concerned with Allah, He may forgive them if He so wills; otherwise one who is guilty of such sins will have to suffer the torments of Hell. He will, however, be sent to Jannah after suffering the punishment if he is a Muslim.