If I do not reveal the knowledge that led me to form that opinion, does that make my opinion less valid?

Your opinion is still valid but it weakens the argument supporting it.

Going back to your original example of which, in your opinion, is the best band:- the term 'best' is not black and white. One band may be best in the studio, another best on stage. A band, that may not be technically the best, may give the best entertainment value. As individuals will put different weighting on these various alternatives, the overall 'best' will always be opinions, no matter how strong the arguments.

DC