The best way to answer this question is to find a balance between insincere answers and answers that draw too much attention to your weaknesses.
It wouldn’t be a good idea to answer with, ‘I am often too good at my job for my own good’. You also wouldn’t want to answer with, ‘I’m not reliable in any way, shape or form’.
Both these answers would have a very negative impact on your application.
The best approach is to find a genuine weakness that you possess, which doesn’t cause significant problems in your daily activities, while showing active measures you take to improve the weakness.
Here is an example: ‘I often feel timid in public situations. As a flight attendant, I am responsible for delivering demonstrations to passengers and, while my shyness has not prevented me from delivering demonstrations, it does make me feel very uncomfortable. In recent months, I have undertaken public speaking classes and I am now growing more confident at delivering demonstrations every day’.