The Most Serious Sin

What is the most serious sin?  It seems very tempting to try to rank sins.  Alma does it a little when chastising his son Corianton.  But I think people often misapply Alma's words.
People tend to excuse lesser sins because they aren't the "more serious sins" in Alma's list.  And they say, "All is well in Zion; yea, Zion prospereth, all is well."  After all, we aren't committing those serious sins.  "And thus the devil cheateth their souls, and leadeth them away carefully down to hell."  (2 Ne 28:21)

I recall hearing that a church leader was asked which sin was the most serious.  "The one that you have the most trouble with," was the response.  I don't know if this conversation actually happened, but it illustrates a very important point.  Even small sins make us unclean and unable to enter into God's presence.  The person with small sins needs the Savior's sacrifice just as much as the person with the big sins.  We all need to repent.  We need to not judge one another, for the self-righteous feelings we get when we deem others less righteous than ourselves will keep us out of any kingdom of glory just as surely as cold blooded murder, unless we have the Atonement of Christ.

Everyone should be working on identifying their own shortcomings and trying to improve.  It doesn't help to compare our sins to the sins of others.  Whether we consider our own problems better or worse than others, it will only hamper our ability to grow.  So what is the worst sin?  The one that will keep us from our eternal happiness.