To be sure it's clear, not the highest roll. Read the guide for the details, but the Margin of Success is the difference between your roll plus all bonuses, and your DDC. You want that to be higher.
Example: Your DDC for a Rule Holding action is 21, while your opponent's is a 23. You roll a total of 24, while he rolls 25. He got a higher roll, but his MoS is only 2 (25-23), whereas yours is 3 (24 - 21). Congrats - you win!