> Can you give an example?

I prefer blondes to brunettes -- that's my opinion.

I prefer chocolate ice cream to strawberry ice cream -- that's my opinion.

You're saying "opinion" when you should be saying "theory" or "hypothesis," both proposals about the true content of reality. An opinion isn't a proposal about reality, it's an expression of personal preference.

> I think I basically consider an opinion to be legitimate if it's plausible-or-better.

But that's not an opinion, that's an observation that may or may not have a connection with objective reality. Opinions by definition are not "legitimate" except in the sense that everyone is entitled to his own. By design, they have nothing to do with reality-testing -- that's reserved to hypotheses, proposals about the content of reality.

"the color green is stupid, it's just blue and yellow mixed together."