Strong and healthy, lots of vigor. If you have 4 females but you really really like #3, make #3 the mother and all resulting clones will share the same phenotypic expressions as the mom.
You'll also want to keep her in a larger pot, because she will spend more time in the soil. If you have the space and plant count, keeping moms is great for when you need to take dozens of clones at one time. But if you are limited on space or plant count and really only need a couple clones from the mom to start the next two plants you'll grow of that strain, I recommend abandon the mother idea and just take clones before you flower the plants. Basically clone to clone to clone.
Whether keeping a mom or going the clone to clone route, you need to keep the plant healthy. If either one suffers from bugs or pathogens, the resulting offspring will be weaker.