On an interview: when they ask what is your worst quality, what is a good answer?

You tell them your worst quality, whatever that is and then you turn it into a positive. You turn it into a positive by telling them that you are very aware of your negative and give examples of situations where you turned it around. Never leave a negative out there without showing the employer that you can turn it into a positive.

I would suggest heading over to http://www.monster.com and review their interview suggestion tips, they've covered this question quite well and can give you far better information than I can remember.

That I sometimes get so absorbed in my work that I work too hard.

You try to do too much or you are an overachiever.

It depends what job you are interviewing for. Ie: you wouldn't want to say your worst quality was public speaking if you were applying for a teaching job. Think of something that can actually be a positive (such as being a perfectionist), or use something that is not that bad and that wouldn't impact the job you are applying for. Stay away from things like: always late, not motivated, etc.

tell 'em the truth, but then add, but i work on it constantly. i try to better myself in that area. they will appreciate the honesty, plus the fact that you're willing to work on it.

I'm a perfectionist!

that you are a perfectionist.

You can say something like "My worst quality is that I get too focused on work at times that I just work and work and forget about other important things in life like watching a movie with friends." That way, it may seem negative but your message is actually positive -- that you like work! Employers love that line.