Definately Snape asking Voldemort to spare Lily's life, because it's a serious part of the essence of the whole story.
Dumbledore is always telling Harry that love is the greatest strength against Voldemort. Especially Lily's love for Harry and her sacrifice. But in the end, although never properly mentioned in the books, it was Snape's love for Lily and his request to spare her life that really made the difference. If Voldemort wouldn't have given Lily the choice and would have just killed her off what he'd surely have done, then Harry wouldn't have lived to become, well, the boy who lived.
I am wondering even that, will they make the Prince's Tale!! the flashback of their death was also very short, but I will be really mad, if they leave it out... but why would be the second part then so long? Yeah, they will make the PT, but it will be awesome or terrible and I hope they choose, a good J and L... not some ridiculous actors...
it would probably be the asking for her life, cus it shows how much he loves her, but i really like all of them! i do love the idea of the mirror erised, but i really love the james and lily one, cus i ADORE the maurders era!