This is the easier question to “what are your weaknesses” except people still struggle to answer it because they are not sure what an employer wants to hear.
The answer is anything! They are looking to see what you consider to be your strength and it doesn’t have to be purely work related. They are also looking to see what your level of confidence is and how honest you are.
The interviewer wants to see how you handle this stressful question firstly then secondly how you handle it. Just be honest.
As with any interview and most tasks you will feel comfortable if you have done your research and preparation, not just about the job and company but about yourself. Can you talk carefully and confidently about your skills? This is what you need to practice.
There are many standard questions in an interview, this being one of them but there are no standard answers and no right or wrong answers when it comes to questions like these.
Make an honest list of what you think your strengths are then select a few of them that you can remember. Practice your responses so that they sound natural not like a script and then you will feel better and able to answer the question.
Also remember that some weaknesses could also be seen as strengths such as: I am a workaholic, I work all of the time. You might view this as a weakness but an employer will probably view it as a positive.
Just remember to be prepared for any standard question.