You can use unique opening lines geared just to the bride and groom but be careful about using lines that only a few will understand. You'll lose your audience if you do and you want them to listen to what you have to say and not lose interest. Try to keep you best man speech on track. It's easy to get side tracked if you're not careful. It has been noticed that the greatest of wedding speeches have great opening lines to begin with and best man speeches are no exceptions - so, you initially need to emphasize on the best man speech opening lines more than anything else in order to give a compelling start to your best man speech.