If you only have one essential oil in your medicine cabinet, what would it be?

Definitely peppermint essential oil. I even carry a small vial of it in my handbag. It gives a cooling sensation and has a calming effect on the body. Sometimes, when I have sore muscles, I will massage in some peppermint oil and it really helps to soothe the soreness. I haven't tried but it's known to freshen bad breath and soothe digestive issues as well.