Choosing the best man, might be the most difficult part of planning the wedding. You must choose the best candidate, without hurting anyone's feelings. The suitability of the choice for best man, and their ability to perform the tasks at hand is the most important issue. A great deal of responsibility will placed upon him, especially at a large wedding, and it would be unfair to ask anyone to take on the role, if they were not capable of handling the task.
From the groom's point of view, there is also another major consideration; the best man is the main moral support during a very emotional point in his life. The best man needs to be someone with whom the groom is completely comfortable with in any situation.
The best man is responsible for ensuring the groom's well being during the bachelor party and for his punctuality on the wedding day. Some friends might be a great time on a boozy night out, but this job entails the ability to hold one's liquor and remain relatively sober and responsible during both the bachelor party and the wedding reception.
It is also helpful if the best man is on good terms with the maid-of-honor and the families of the bride and groom. If possible, make all of the necessary introductions well before the wedding. Good communication is at the heart of good planning and the better acquainted the main organizers are, the better they will work together.