This is a hard question, and you should prioritize it in your efforts. The key is doing the proper introspection. Think about past experiences that point to specific strengths.
With regards to examples, my #1 message about this essay is DO NOT TALK ABOUT WORK. They mention *personal* like 9 times in the prompt. This is the #1 error that most people make. There are so many opportunities to talk about work elsewhere in the INSEAD application.
One of my R1 candidates has a past history of organizing groups or initiatives that help others achieve a dream they felt was impossible “on the cheap” and provided 2 quick anecdotes to support that. The mantra is show don’t tell. Make your claims, but make sure to illustrate this with examples (for both strengths and weaknesses.)
If you don’t have a lot of clarity around your strengths and weaknesses- survey your friends and family and ask for their input with this. I use this tool, it is free for 15 days. Make sure to let everyone know you need feedback rather quickly. http://www.reachcc.com/reach/survey.nsf
The strengthfinders assessment is also very useful, if you can support the results it with personal anecdotes. If you buy the book it comes with an assessment. http://www.strengthsfinder.com/home.aspx
And finally, there is a values quiz on MindTools https://www.mindtools.com/pages/article/newTED_85.htm
The key is to reflect on past times in your life when you did well (and not so well) and trace back the strengths and weaknesses those point to. The objective of this question is to give them insight into who you are, not just what you have done. And from this, they can discern your level of self-awareness. With this question, you want to make an emotional connection with the adcom. So often applicants spend all their time discussing accomplishments but are afraid to open up and describe who they really are as a person.
When someone is candid about their failings, it opens our heart to them, and we feel more connected to them. The importance of making an emotional, heartfelt connection to make you memorable cannot be overstated.