This position holds the most prestige -- and the most responsibility. Not only will your best man plan your bachelor party, he'll also keep the groomsmen in line and on time, hold onto your bride's wedding band, and give a toast at the reception.
Most grooms select a family member for this job, but you can also tap a close friend for the privilege. Besides choosing someone you care about and have a history with, the important thing is to make sure your best man is someone you trust. After all, you don't want him forgetting the ring or showing up late to the ceremony!