You should claim your greatest weakness is your job-oriented obsessiveness ;-)
Chocolate works too.
The hardest ones for me are the negative ones.
Like another example is "What did you like least about your last job / employer?" Um...not easy to answer. To begin with, I try not to dwell on such things, so I tend not to remember 'em. And secondly, if there WERE bad things (pay was a big one...) how do you say anything without sounding petty and hypercritical?
I guess I hope that's one that doesn't get asked.