It's hard to say between peak fire bending and peak water bending. Lightning has proven to be deadly if it strikes you. Aang would have died if Katara hadn't healed him immediately. On the other side of things, blood bending is insanely deadly. Being able to immobilize your opponent and cripple them is scary. I would say lightning wins out though because it is easier to access... Unless you're Tarrlok who can blood bend at will.