'the most important'? 'what's most important'?
I'm not sure how to construct a natural-sounding sentence with 'priority' in this case (suggestions welcome!)
OK, thanks all! I have edited following handyy's suggestion. As for the 'extra' 'in', this is required I reckon. We are talking about things that are important in the world in which we live; 'which' can and is often left out, but 'in' has to stay. We don't live this world, we live in this world