Question: I am currently working on at least five stories that I think all have potential, but I think that I need to concentrate on one or else I'll never get finished. How do I choose, if at all?
Writing a novel can be a long process that can take anywhere from several months to several years. So you're absolutely right that you need to pick one to concentrate on.
I personally find that it's best to be able to focus on one story at a time so that the details of the story remain fresh and alive in my consciousness throughout the writing process. If you switch back and forth between several stories, you have to spend more time re-acquainting yourself with where you were.
(That said, there are occasions when, if you are absolutely stuck on a story, it can help to cleanse the pallet by working on another for a little while.)
As for how to choose, if you have one story that feels like it wants to be written ahead of the others, that makes it easy.
If they all are equally pressing, you may just have to pick one.
Some factors that might make a difference are...
1. Which one will be easier to write? Which requires less research or is more manageable?
2, Which is more marketable? If you sell some stories, you will have an easier time selling others that appears less marketable later.
3. Similarly, is one in a genre that appears to be heating up? If you can write quickly, you might have an easier time selling it.
4. Have you already done a lot of work on one story? If so, why not finish it, so it's no longer pressing on you?
5. Is one more topical? Is there an event that will take place in a 2-3 years that will tie in nicely? For example, a key anniversary or celebration? If you can get your book ready to launch by then, it could help sales.
6. Recognizing that working on a novel requires a considerable investment of time, you might ask yourself which idea you will feel better/prouder for having completed.