On some things, everybody is a liar and everybody is a hypocrite. Those two things can't be escaped completely.
For me, a lack of empathy is probably the most serious flaw. Not just a lack though, as we all lack enough empathy on certain things. An unwillingness to empathize is probably the worst flaw. This is usually the result of arrogance on their part, which is a close (if not equal) second.