In the novel Frankenstein, what is Victor most guilty of?

Excellent question!  Victor is ambitious to a fault.  His grandiose goals include becoming famous for something that no one else has ever done.  He begins with the idea of being able to save those we love from certain death--a nice thought if it meant we wouldn't have population problems, etc. later on down the road.  Victor fails to think things through carefully--the consequences are never considered--he just goes at his ideas and goals like he's killing snakes.  He wants to create life, and he does.  He never once considers who and how it will be cared for or what's do be done with it afterward.  In fact, he never thinks of it as ugly until it breathes life.   

His irresponsibility coupled with the ambition gets my vote for his greatest fault.  Had he considered anyone other than himself and how others would be effected by his actions and inventions, the book would have been a very different read.