The Tiger rattlesnake takes the prize for the highest toxicity of venom in the western hemisphere and produces a neurotoxin essentially identical to the Mohave rattlesnake along with a myotoxin that causes muscular necrosis. However, the Tiger rattlesnake is notoriously reclusive and reluctant to strike unless intentionally provoked. Most people will go their entire lives in southern Arizona and never see one. As of this date, I have only encountered two in my entire career!
Wikipedia - Tiger Rattlesnake