Killer Questions

When I am preparing a new survey there are often one or two questions which I usually include and which invariably provide great feedback. The one which always seems to work is

'What is the one thing we can do to improve the service we offer?' This has provided valuable, actionable information for many clients.

Last year one client, against my advice, changed the wording to 'What are the three things we can do to improve the service we offer?' - thinking they were being really clever and would get three times the information. In fact they got no useful feedback at all.

Questions should be clear and easy to answer - this was asking too much of respondents!

What are your killer questions?