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.
Lavender essential oil as its smell good and able to calm down one's too
Lavender for me - so I can have a goods night rest ;)