This interview question -- "What people do you find it hard to work with?" -- is a tough one. It's tough because it is asking you to speak negatively about some group of people. You never want to go negative, but here it is.
Since you don't want to go negative, you have to answer with situations where it was hard to work with some people and what you did to work with them. If you don't answer with situations, you risk alienating entire classes of coworkers. Wrong answer.
Let's look at this question a bit closer.
This is a corporate culture question
This question asks you to describe a work environment that you really don't like. It is searching for the answer to "will you fit in with the team."
Consequently, you need to know the type of environment in which you work your best. Confrontational? Collaborative? You will need to contrast the corporate experience you like with ones that you don't.
Assumes there are people you won't like in any group, but need to work with them
Every group has individuals that other individuals have a hard time working with -- no one has the exact same personality and compatibility; indeed, some diversity of opinion in this area is good.
Good managers know that business is social and look to see how potential hires will work in a situation where they don't like someone.
Focus on the work, not the personality
You start describing your situation using the CAR methodology -- Context, Actions, and Results. You can talk about a situation, but the action you want to take here is to "focus on the work" and not on the personality. If you ever focus on the personality, it gets you into attacking people and not their ideas.
By focusing on the work, you gain a larger sphere of people you can work with because the work is what matters, not, as much, the person.
Focus on understanding their point of view
One of the difficult things about working with difficult (to you) people is that you hear what you don't like about them rather than what they are trying to say. What they may be saying, even though you don't like how they are saying it, may be fabulous stuff that can really help the situation. By taking the action of understanding their viewpoint on the work, you will go along way to ensuring they feel heard.
Focus on making the situation resolve in a better way than your way
Inclusion of good ideas from multiple people should make any resolution or approach to the work better. By focusing on finding the best solution through listening to other people, you will often come up with a better approach. Taking action to find the better way shows you can focus on the work and not the person.
Don't forget the Result
Once you've described the situation and good actions, you need to tie it all up with results. "Because of this focus," you could say, "we came up with a completely different solution than what we started with. And even though it was hard, that situation resulted a lot of respect for each others approaches on the team."
Hiring managers want results oriented people, so make sure you tie this together with a satisfying, results outcome.
Who was the most difficult person you've had to work with -- and how did you work with that person?