That's your opinion
Saying "That's your opinion" (or other words to the same effect) in response to a reasoned argument is basically a form of dismissal that ignores any objective, rational case the defender may have made.
It is similar to the "That's your bias" attack.
- 2009-03-29 The Ethics of Framing Science: Four Guiding Principles "...pundits such as Richard Dawkins use their authority as scientists to argue their personal opinion that science undermines the validity of religion..." – totally ignoring the fact that Dawkins made a rational, evidence-based case for this conclusion. The assumption seems to be that certain issues are definitionally "matters of opinion", and therefore any position taken on those issues must also be "opinion" regardless of what the facts might be.