Well, don't make it up and don't use the old stand by, "I'm a perfectionist". Damn, I hate that one. It's so old.
But, try to think of your worst quality that also has a possitive spin on it. If procrastination is your worst, go ahead and just skip that one!
My worst thing was the flip side to my best quality. I said that when I focus on something, I'm so caught up in it that other things in life tend to go to the wayside. I had a hard time balancing all the important things in life.