Pride. Seems simple, but I think people underestimate what the fall really was for humanity, it came down to pride just like the previous angel fall. Eating an apple to know as much as God is pride,(they trusted themselves and the snake more than God). Thinking you have authority the way God does, pride. Thinking you can do whatever you want with no eternal consequences, pride. All of our problems can be broken down to pride I believe. The other sins are bad but this one seems the hardest to break us from.