I would answer like this:

Hmmm, honestly I don't know how to answer this question. I am a hard working person with modest personality. So far up to now I have been getting along with my coworkers very well and the projects I've worked on are all very successful. If I really need to think about my weakness(here pretend you are thinking), it seems ... like sometimes I may be too modest, as sometimes I have to work overtime without telling others to cover some very little, very detailed things that the team overlooked to make the projects successful. Since I am not telling, I don't get credits for these overtime work.