Answer: For immediate relief I would probably choose a cup of catnip tea, brewed with one teaspoon of high quality, dried catnip per cup of boiling water, steeped about seven minutes. If I were traveling, I might choose a quarter teaspoon of catnip tincture dissolved in a half cup or so of water, although I don't think the tincture is quite as effective as cradling a cup of tea and feeling the warmth permeate your whole body. If catnip didn't work for me for some reason, whether because the pain was too severe or for some other reason, I would consider some of the suggestions in Susun Weed's book Down There, such as motherwort tincture, crampbark or willow bark tincture.
Those are all things I would try for temporary relief but from my experience there are other things a person can do for long term relief from menstrual cramping. I used to have moderately painful cramping before drinking oatstraw infusions on a regular basis. My last period I felt the tiniest sense of tightness in my belly the day before I started and that was all. I also drink red raspberry tea the week of my period and I find it both strengthening and soothing. In general I try to eat mostly whole foods prepared at home and this seems to decrease menstrual bloating as well as cramping.