I don't think that it's one particular sin , after all Jesus starts by saying " every kind of sin CAN be forgiven , but anyone who speaks out against the Holy Spirit will not be forgiven ."
However to have a sin forgiven you have to repent and ask for forgiveness . In 1 John 5 : 11- 12 we are told " God has given us eternal life , and this life is in his Son . Who ever has the Son has life , who ever does not have the Son does not have life "
As many have pointed out Jesus made his statement to the Pharisees who were accusing him , basically, of being in league with the devil .
Therefore they WILL NOT listen to his teaching , repent of their sins , ask forgiveness and receive Him into their lives as saviour . So they are rendering themselves unforgivable.
Jan