In negotiation scenarios what is the most difficult you have been involved in?

The interviewer may well be looking for an example of your ability to influence the outcome of a work related situation or problem. It's perhaps diplomatic to find within your achievements an example that demonstrates your ability to engage with and encourage people to contribute to a win-win situation. Don't be tempted to cite technical examples that demonstrate you have greater knowledge or expertise than others involved in the discussion but rather demonstrate more gentle and persuasive manner which demonstrates that you are able to engage and compromise to encourage a resolution as far as possible that is acceptable to all involved.