Simply stated

Read James 5:12
James reflects Jesus teaching and urges the Christians to keep their interactions with one another clean, honest and uncluttered by exaggerated promises. Why do you think he started this verse with, “But most of all”?

The way I read this verse, "but most of all" indicates that James is placing this teaching as the most important thing for the reader to remember.  So, he's saying that refraining from swearing is more important than:

• being patient for the Lord's coming, and in the meantime, preparing our hearts
• not grumbling or blaming others

Above all else, we are to refrain from exaggeration and swearing.  By simply stating what we are saying, we will be seen as honest and truthful.

I have to admit - I struggle with this question in understanding why he places this as the biggest thing to pay attention to - above patience, preparing our hearts for the Lord, and not grumbling.  How do you feel about this?