What would you name as the one character flaw that is most dangerous.
I'd say lack of respect for other people. It comes in many guises and it's always bad news. I think it's the root caase of many of the other flaws that have been or will be nominated. Intolerance, pride (the wrong kind!), petulance, bad manners - they all boil down to lack of respect.
And I don't mean that old-fashioned doff-your-hat and "respect" your elders/betters. Elder isn't necessarily better and neither is economic status.