Side step to knee. Weakest point of your leg from hip being point "A" and your foot being point "B".It's hard to break a hip and to hard to break someones ankle while standing. If your on the ground there are many ankle breaks that Jujitsu offers, the grapplers love to crank that ankle, it doesn't take much to break either. But your question was the weakest part of the leg and in my opinion that would be the knee.
Your other question was "All the weakest spots" on the body, that a big question dude, buy one of those charts that shows all the striking points, usually every Dojo has one hanging someplace.