Another blog I read asked this question the other day. How you answer tells you a lot about how good you are at promoting yourself (a trait that will become very important as we advance in our careers and start job-searching, etc.). For example, if you preface your answer with "I think I'm good at..." you make yourself sound weak.
The blogger also noticed that very few professors in her experience would answer this question with "I'm a good teacher or mentor" - even if they had won awards in the past for teaching! I think how we answer this question says a lot about our views on academia and our place in it. If we believe research is the most important part of a professor's job, then of course we won't list "teaching" as one of our better skills for fear that doing so would make us look like we have nothing else to offer.
When I thought about how I would choose to answer this question, I realized my answer would change depending on whether my lab(and blog)mates were in the room. I apparently feel more comfortable building myself up as a researcher when people who have actually seen me research (or not) are not in the room. This shocked me and is making me re-evaluate what it is I'm good at.
These "tips" aren't news - we should all be capable of recognizing that qualifying our answer is a no-no. It's hard to admit that we are really good at something useful, and not just "getting by" or "messing around" or something. I encourage anyone who reads this to think about how they would answer this question (and if you feel comfortable, post your answer in the comments section so we can marvel at what wonderful readers we have!)