Anyone who tries to convince you there is no realpolitik to choosing your groomsmen is not to be trusted. Having spent six weekends this year at weddings of close friends of mine, the first year with so many weddings, what I learned is that being invited to a wedding is wonderful, but being asked to be a part of it is even better.
If there is one guarantee, it is that at your wedding, there will be one or two friends there who feel like they may have been in the running for it but didn't pass muster for one reason or another, all the while looking around before the service starts at the uniformed groomsmen with oversize (not at my wedding) button holes pinned to their lapels designed especially to make them stand out.
What is important is to honour the families involved first. I happen to very much like my fiancé's brother, but understand this is not always the case, suck it up, they are there to help. Have all your brothers too if applicable then begin the more difficult choice of which friends you have. Try if possible to match with the bridesmaids. I managed to not do that, I have five brothers, one future-brother-in-law and only one male cousin I didn't want to leave out. So before I had gotten onto my friends, I had outnumbered the proposed bridesmaids. Nevertheless, 4 ushers are extra-familial, and one master of ceremonies, a choice that was very much collaborative.
It is extremely important that you tell them what to do. At the rehearsal, keep the instructions clear, divide up the jobs (for as groom, this will be the only chance to delegate), and make sure they know how and where to seat people. Families get very sensitive if they show up and the ushers are not sure who they are and are not clear on where to put them. This is key. If all the above criteria are met, your team will be invaluable, and if it rains, will be the playmakers for keeping people's spirits high. A Note on the Best Man: Do you have a best friend? If you do, he may not be the best choice for best man. If you have a best friend that is extraordinarily organised and can speak in public confidently (speeches will be in a different entry) then that is your man. I am lucky, very lucky. I have a twin, a twin who is efficient and brilliant and managed, unlike me, to nurture a sense of humour.
None of my friends or other siblings ever really saw themselves in the running. Whoever you choose, I suggest you pull them aside nice and early and explain two things clearly. 1. What you might want people to spend on a stag do. 2. That stitching your mate up on his wedding day hasn't been funny for a very long time and makes people uncomfortable.