This is a personal question and you should generally just be honest about your greatest strength, but make sure the wording is right. Passion, attitude, leadership, intellectual curiosity, work ethic, resilient; the list goes on. Don’t say something that makes you seem cocky, like intelligence (everybody in IB is intelligent to some degree).
Ideally, you want to give an example of this strength by telling a story, either related to work or extracurriculars. For example, if your greatest strength is your work ethic, tell a story about how you juggled multiple priorities and projects such as work, school, extracurriculars, etc. Provide the context for why you were juggling so much and the steps you took to prioritize the most important projects, as well as how you communicated your progress to your manager or other stakeholders. Finally, conclude with the result of your actions and how it positively impacted the organization.