What was the hardest job interview question?

Here are some data points. Describe some algorithms that can be used to categorise them?

Tell me about yourself.

Any questions?

"Why do you want to work for us?", if only "Because you'll fucking pay me" was an acceptable answer.

@smithalan2
I've done my share of (technical portion) interviews. The HR or the manager would *always* pull this question out. I've often wondered what a neck of a question was. I asked the HR manager and he told me the usual blatant stuff about matching of corporation vision, objective sharing and bla bla. Well, bullshit, give him a goal and see if he can deal with it.
Then I asked my manager.
He simply answered me: "well, if he is interested in the company and in what we do here as a whole, maybe we won't train him for 9 month to see him flee away right after".
Made sense to me.

The question the interviewer asks when your brain is frozen with fear

A friend of mine and me had interviews at the same company sequentially. They asked: "would you recommend us your friend?"

What are your weaknesses?

What salary are you looking for?
Always the hardest.

Why shouldn't we hire you?

@Ashkin applied at a startup (15-20 people). I did not know what salary i should say. It was a back an forth. I waited until they told me what they usually pay.
Finally got an offer: It was just a bit higher than every junior dev gets... I knew that startup pay badly, but this was way under my expectations...

@dprinz my current job is with a tiny indie game design studio. I love the work, but the pay is peanuts.

(Actually, I can't even afford peanuts...)

@PenguinRage I hadn't, it wasn't, it was off the cuff statistical analysis that I hadn't done since A-level/high-school (7 years).