First let's define what a value proposition is:
"A value proposition is the key differentiator that you offer compared with your competitors."
Here is a link to a great info graphic all about value propositions.
A key factor to remember is that your value proposition is one of the top reasons why a customer will pick you over your competitors.
What a value proposition is vs what it isn't
Checklist for great value propositions:Can it be understood in the first 5 seconds? Is it clear? Does it explain why it's different and better than your competitors?
Examples of effective value propositions: