What are the best question to ask a committee panel after a PhD interview?
It is not mandatory to ask questions. Generally they offer you the opportunity to ask questions in case you are not sure or confused about anything or any process related to application/lab/university. 

When I had given PhD application interview almost all times interviewer asked me if I have any question. I didn't ask questions for most of the time. However few times I do asked question specially when position they were offering had partial financial support from supervisor. That case I asked about chances of getting rest of financial support. They were quite helpful and actually suggested me to check for RA positions in other related departments. In another case, I asked professor about possibility of collaborative work because I was interested in some interdisciplinary work. He was very impressed by question as it showed that I had already focused area of interest. 

I also remember few of my friends have asked questions like, "Are there many flowering plants around campus because I am allergic to pollen-grains?" , "Can I get accommodation on campus if a dependent is accompanying me? ", "You were working on XX problem before are you still working on that?" etc.

You should ask questions depending on university you are applying and professors interviewing you. Do some basic research on their background from university web page. Don't ask cliche questions and don't ask for the sake of asking.