How do you decide which past accomplishment is worthy of a 30 minute conversation? Usually what comes to mind when asked this question are simply the most recent things I've worked on. If you've had the misfortune of doing crappy work for the last few months then it makes it a bit harder.
I've seen lots of these kinds of posts, trying to "solve" interviewing. It's a topic close to me right now as I'm in the middle of the interview cycle myself. I just don't think it can be solved. There are too many variables and too many non-scientific aspects to it. Most of the time interviews are decided in that first impression with your interviewer - it feels like a primal instinct people have about other people.
One thing I can say though, is that you aren't going to find the perfect candidate with just one question.