Also think about what do you use at the moment anyway? Then if you can plant those or herbal alternatives to what you use, you know that you're building up a practical herb garden medicine cabinet, rather than plants that you're less likely to use.
What illnesses do you and family already have, or are common to your area? For instance, cold, flu, headaches...
Colds and flu are treated in my home, with boneset, mullien and goldenrod (for best taste, use before flowers 'puff') teas, mint helps with taste as well as healing, preferably with honey to sweeten. Oregano would be an excellent addition. It does not like to grow for me, so I buy capsules. My headaches are treated with wild violet flowers when available, or dandelion root.
Also, find plants that are not a drain on your lifestyle. I thought I needed to have a citrus fruit for vitamin c. This requires heat to survive my winters. Pine needle tea has lots of vit c. No tending needed. Or Rosa rugosa for rose hips, lots of vit c, minimum tending. Find out what "weeds" have your medicine and are naturalized in your area. Plant them in your herb garden. Pasionflower is a weed hereabouts. Useful and beautiful. http://medicinalherbinfo.org/herbs/PassionFlower.html
http://medicinalherbinfo.org/index.html This site lists herbs, what they have been used for, where in the US they grow naturalized. Also has an area cross referenced with what condition or disease has been treated with what herb. All with footnotes.