What is the strongest allergy medicine?

Nasal spray. The best medical regimen for treating allergies would be a combination of a nasal steroid and a nasal antihistamine.
Steroids. Depending upon what you are treating, steroids in one of many forms is the usual standard of strength. The best non steroids for effectiveness are Benadryl (diphenhydramine) and Chlorpheneramine. There are some high power drugs in the anti-cancer family which can be even more potent but are used only as a last resort.