An opinion on something subjective- i.e., I think kobe beef is the finest meat in the land -cannot be wrong. It is a personal opinion whose basis of fact and statement stems purely from one's own ideas and experiences. Nothing, save the outright negation of a person's beliefs and ideas, could make such an opinion wrong, even if its an opinion held for only as long as it takes to SAY the opinion. I like the color black, I prefer playing GURPS to playing D&D, and to me, the best soda in the world is Mountain Dew Code Red. Because these are purely subjective statements, unless you can invalidate my very mind and existence, you cannot claim them to be wrong, anymore than I could claim you are incorrect in the things you like.
An opinion on something quantifiable- i.e., I think the moon is made of cheese -is NOT AN OPINION. It's a statement on an actual fact. You don't have 'an opinion' that your jacket can stop a meteor or that your cat is fifty feet tall and breathes fire when no one is looking, you have an incorrect understanding of factual data that can be accurately measured. And whenever someone says something amazingly stupid along the lines of willful ignorance about a religion, race, political party, philosophy, nationality, or what have you, they tend to use this 'definition' of an opinion. It's not your opinion, it's your excuse for being an idiot. Educate yourself, get some first hand experience to where you actually understand the shades of grey in the world, and then- THEN! -you can have an opinion based on personal likes and dislikes, ideas, and experiences. And at that point, I might disagree vehemently, but it'd still be YOUR OPINION. And by that, for you, it couldn't be wrong.
That said, the statement, "Supernova was the worst sci-fi film of all time" is not opinion. It is quantifiable fact.
[runs away after invalidating his own wall of text]