Fox Labs pepper spray seems to have the strongest concentration
5.3 million SHU (Scoville Heat Units)
A lot of pepper sprays use a 1-2 million SHU formulation.
You can purchase
Fox Labs products
here and
here along with
information about
Fox Labs pepper spray here.
Another question would be "Do you want a
stream or
fog spray pattern?" I would recommend a
stream pattern as it stands up better in windy conditions and offers less blow back onto the user than the fog patterns do if aren't paying close attention to the wind conditions.
A word of Caution: OC pepper sprays are proven safe with no known adverse reactions. Combination sprays that use OC along with a blend of CN or CS chemical tear gas type products have been known to cause allergic reactions. Also some states such as Michigan, IIRC state it is illegal for civilians to use chemical sprays that contain dye that will stain your assailant for later identification.
Just be sure and use due diligence when you look into buying pepper sprays and check your local laws.