This was supposed to have gone up a week ago. I don’t know why it didn’t.
A lot of us writers have it easy. We have one strong character idea and are ready to center a story around them, or we have a strong plot idea and just need to design a character who can handle it. Sometimes, though, you have a nice little group of decently developed characters—ordinarily a good thing, as there are few stories that cannot be improved by increasing the proportion of well-rounded, well-realized characters—but no idea which one to use as the central or the viewpoint character, and not enough of anything else to provide a tiebreaker. It’s an easy spot to get stuck at, but fortunately, there are some useful tiebreakers out there.
One trick is just to choose a situation that you expect to come up, preferably something at or near the beginning of what plot you might already have, and start writing it or plotting it from their different perspectives. This gives you a chance to compare how they approach problems, how they feel to write, how what they know affects the storyline, and just who’s the most interesting to narrate through; maybe you like this one’s voice, or you’ve found that that one makes decisions that make no sense unless you’re in her head. And sometimes, one of them will just reach in and take a story over; I had one project where the character originally designed as the original main character’s love interest got embroiled in the plot and subsequently demonstrated that her part of the storyline was a lot more interesting.
Another, if you’ve got a somewhat developed storyline, is to look at who would be the most convenient—or eliminate whoever would be most inconvenient. If you’re going to get suspense out of one particular secret, for instance, you’ll want to avoid the person who’s perfectly situated to already have the answer; if, however, you expect the opening of the plot to trade on a certain piece of knowledge or motivation, it would be silly not to focus on a character who would know or care.
If constant improvement is one of your motivations, you might also consider trying the one who is most on the edge of your comfort zone—not so far out you can’t write them, mind, but enough to make you stretch your abilities a bit.
Another thing you can consider is who you’re writing for, and whether any of your choices would have a particular advantage with the audience you want to appeal to. I definitely wouldn’t make this the first tiebreaker choice, but if you’ve gone through one or two of the other questions and are stuck between a list of about two or three strong characters, it mitigates the risk of having a main character who is audience-appealing and practically nothing else.
The big thing to keep in mind is that, unless (and even if) you’re a compulsive pre-outliner, you don’t need to make this first decision your final choice. If you choose one character now, but another demonstrates herself more interesting while you’re halfway through, it’s probably worth considering going back and revising her into the main character seat. Until it’s somewhere in the publishing phase, everything can still be fluid.