In case of the world ending scenario, I would run back for nothing. But watch everything within my vicinity wither, because there is a beauty in destruction. Why miss the show when the end is imminent and inevitable.
And if talking in general, as to what really matters, my answer is Integrity.
It's hard to chose one thing... so I'd probably chose the dearest person to me, and I can chose only one. What's the use living with only one thing, which may die, or rot, or be ruined after some time... I don't know what I'd chose.