A very good question indeed! A few years back, when I had switched my job, I had been asked the same question by my interviewer. A perfect answer to this question usually depends on the challenges experienced by an individual.
Software Testing has a lot of challenges both in the Manual as well as in Automation.
Discovering a bug that was difficult to be found out could be the answer to this question. It was quite challenging for me. Fixing these un-reproducible bugs can be difficult but is often possible.
Always keep in mind that whenever you are answering such a question, be realistic and don’t stress. Do not let anyone dull your sparkle. Just be yourself.