Nature is so generous that she provided us with thousands of herbs to choose from, all of them with medicinal properties. It is difficult to pick just one or three!
The ones I use more often, just because they are very accesible, are rosemary, thyme and lavender. I know MH will tell you that Olive Leaf is powerful, and I agree as well. My instint tells me it will be the King of all herbs for me once I have the chance to experiment with it.