View from the top,
As Mungo Man said, honesty is the best policy.
I was asked this question when interviewed. To give you food for thought, you could mention things like, you liked the way the company operates, and are interested in their expansion plans. The fleet they operate appeal to you and you like their destinations route structure, maybe mention cross conversion oppotunities, being dual rated etc. The base(s) appeals to you, the company has a good reputation etc, etc.
There is no wrong or right answers. Also be prepared for 'so you have 30 seconds to sell yourself, why should we pick you?'
Good luck with it.