If sins are to be ranked, denying the Holy Ghost is the worst. This sin qualifies one for an eternity with Satan as a son of Perdition (DC 76:31-38). The sin of murder is next. Those who commit pre-meditated murder cannot be forgiven and will inherit a telestial glory (Rev 22:15, DC 76:103). Adultery is next on the list. Although forgiveness can be obtained, it is not easy.
(Applicable to our day, is the sin of elective abortion. One might wonder where this sin ranks in the list of most abominable sins. While the brethren have not specified its rank, it is clear that this sin is at least as serious as adultery. The brethren have consistently used the following scripture to refer to the sin of abortion. It states, Thou shalt not steal; neither commit adultery, nor kill, nor do anything like unto it (DC 59:6, italics added). The fundamental element of the sins of murder, abortion, and adultery is man’s tampering with the power of life itself)
"What is the difference between an ‘unpardonable’ and an ‘unforgivable’ sin?
"The second most serious sin is to commit murder -- that is, to willfully shed innocent blood. Concerning this sin, the Lord has said: ’Thou shalt not kill; and he that kills shall not have forgiveness in this world, nor in the world to come.’ (D&C 42:18.) Thus this is an unforgivable sin, which means that Jesus Christ cannot pay for (or ‘atone for’ or ‘forgive’) the penalty demanded by the broken law. This sin is a pardonable sin, however; that is, the sinner can eventually make full payment himself, and be received into a state of pardon. Apparently one reason this sin is unforgivable is that forgiveness is based upon repentance, and a murderer cannot fully repent of his sin for he cannot make restitution of the life he has taken.
"The third most serious sin is unchastity. The Lord has said, ’Thou shalt not commit adultery; and he that committeth adultery, and repenteth not, shall be cast out. But he that has committed adultery and repents with all his heart, and forsaketh it, and doeth it no more, thou shalt forgive; But if he doeth it again, he shall not be forgiven, but shall be cast out.’ (D&C 42:24-26.) Thus adultery is both pardonable and forgivable, but if committed again after a person understands the law it is unforgivable…
“All unpardonable sins are of necessity also unforgivable.” (Daniel Ludlow, A Companion to Your Study of the Book of Mormon, pp. 221-2)
Bruce C. Hafen
“Perhaps there is a common element in those two sins, unchastity and murder. Both have to do with life, which touches upon the highest of divine powers. Murder involves the wrongful taking of life, while moral transgressions may lead to the wrongful giving of life. At the least, sexual transgressions involve a wrongful tampering with the sacred fountains of life-giving power. Immorality rejects the God-given sanctity of life itself.” (Belonging Heart, p. 271)