There is no right, or wrong, reply to this question. The interviewer wants to get to know you, and what your expectations may be when it comes to a luxury experience. When a question is open-ended like this, it is easy to ramble on, so be sure to stick to a few key points. For instance, discuss where in the world you would go, where you would stay, and how you would be treated. Perhaps you can mention which amenities would be available to you, on this ideal luxury holiday.
"I would love to stay at the Emirates Palace in Abu Dhabi. Did you know that the project cost $3B to build? I have read about how exquisite it is, with a thousand crystal chandeliers throughout the property, fountains, and pools everywhere, and the most luxurious spa. I would purchase their 5 1/2 hour spa day which includes a steam body cleanse, pedicure and manicure, full body massage, and a sea salt exfoliating ritual. To me, it sounds like the ultimate luxury holiday."
"Staying with Kempinski Hotels S.A. sounds like a luxury vacation stay, to me. I have not had the opportunity to travel often in my life so the chance to stay here, and experience your luxury spa, fine dining, and relaxing pools would be an incredible experience."