Fight the urge to say there's nothing you dislike. While this is the perfect answer in theory, in reality if falls flat because it's simply not true. No job is perfect, so pick something that everyone can relate to. Some part of the restaurant business that nobody likes. That way you are being completely honest in your answer, but without sounding undesirable. Here is an answer that any restaurant worker can relate to:

"My least favorite part of serving is bad tips. Not every customer will appreciate my service as much as others. But it's not something I can change, and I try not to take it personally. I can't expect every customer to tip the way I would."