"What single project or task would you consider your most significant accomplishment in your career to date?" If this is the best question you can ask people, we got really lucky, because this is the essential question on all YC's application forms for events (e.g. Startup School) and has been one of the core questions on the YC application since the beginning.
On the other hand, in my experience, the question works much better when asked in person than in writing. Just seeing the face of the interviewee while you ask the question lets you understand if there is real passion for that kind of job; the answers I get in writing these days (ie by email while screening people) are unfortunately much less telling :(
This is a subset of the fact that the employment paradigm, with its multiple perverse incentives for all players, requires us to be liars. If we're very lucky, we just have to know how to be a liar.