Generally speaking, everyone wants to be involved in your wedding and your friends all want to be part of your official wedding party. For men, the decision boils down to the Best Man. Women can cheat, a little, having both a Maid of Honor and a Matron of Honor. Even so, that's only two people. In effect, you're telling everyone else that, "You're my friend, but you're not my best friend!"
Sort of sucks, eh? When you throw family into the mix, things get even stickier.
Some people seem to feel "entitled" to the position. In my opinion, you axe them immediately...but that's just my opinion. Entitlement annoys me.
Beyond that, you shouldn't necessarily pick the person you like the best (don't lie, you know that you can pick your favorite!), but you should pick the person who will be the best in the position. In many ways, you really are interviewing for a very important imposition! The difference is that you don't conduct the interview, you base it upon information you already have.
Let's start with the woman. Before making a decision, make sure she isn't heavily in debt or living paycheck-to-paycheck. Traditionally, the maid of honor has a lot of responsibility...I mean, just look at the following list!
Your maid of honor is going to be responsible for helping the bride shop for a gown, pulling off the bachelorette party, pulling off the bridal show, take notes on the gifts received, keep the bridesmaids in line and on time, keep the groom's ring safe during the ceremony, hang on to the bouquet, sign the license as a witness, dance with the best man during the first official dance and generally provide a toast to the new witness.
As you can see, you need to a pick a woman with her head squarely on her shoulders, the ability to prioritize and manage tasks and the ability to stay calm under pressure. Evaluate prospective Maid's of Honor according to those concepts, not simply because you've "known her forever."
Now we'll move on to the men, who supposedly have it "easier" by traditional standards. I mean, all the guy has to do is show up in a tux, right? Wrong. The Best Man has to be dependable and reliable and capable of keeping a cool head under heat. Mainly, the Best Man is going to be responsible for keeping the other groomsmen in line (in a nice way), throwing a bachelor party (which doesn't get the groom in trouble) and helping keep the groom from getting cold feet.
Perhaps your existing "best friends" are suitable to this requirements, perhaps they are not. Don't suck into tradition which says that your best friends must fill these roles. Be pragmatic about this decision so that your day is less stressful and more enjoyable.