I think all sins are forgivable, but I think the more serious the sin, the longer and more good you have to do to redeem yourself...like child molestation, being mean to seniors and children. Just because Jesus is willing to forgive you, doesn't give you permission to sin and do it again.
Blasphemy against the holy spirit is mentioned in the Bible as an unforgivable sin. Adam and Eve, Judas Iscariot and some of the pharisees of Jesus' time are considered to have committed unforgivable sin.
I'm not accounted for as a pure Christian, but what unforgivable sin I would have to say is committing murder or just pretty much breaking any of the Ten Commandments too. And also if a person who has been saved and then instead of living right for the Lord just would rather turn back toward their ways of bad living thinking that just because they were saved that it's going to help them, but truth is NO it's not going to help them they must stop living with whatever bad ways and try to live the right way instead....
There are multiple verses in the Bible in which Jesus speaks of the unpardonable sin: Luke 12:10 - And everyone who speaks a word against the Son of Man will be forgiven, but the one who blasphemes against the Holy Spirit will not be forgiven. Mark 3:28-29 - Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin. Matthew 12:31-32 - Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. (My answer continues in the comments)