I agree with you that you shouldn't tell anyone their opinion is wrong, or worse. However, the lines aren't always that clear.
first of all, many people conflate opinion and fact. The earth being round(ish) is a fact, not an opinion. Your (not you, personally) opinion of it being flat doesn't make that true.
And second, while taste cannot really be judged other than by standards set by society, there are qualities in movies, art, most form of media, actually, that can be sorted by quality.
Can you enjoy the movie 'The Room'? Yes. And anyone who tells you you can't is wrong. Can you say you think it's better than The Godfather? Yes, sure. Is it, factually, better? If you ignore all factors that are down to personal preference, the subjective ones, and just look the objective factors, like acting, set design, soundtrack, cinematography, you'll be hard pressed to find anyone who'd agree with you if you claimed The Room did any of those better than The Godfather.
The middle ground, however, seems to be a lost or dying art. No matter how feverishly one enjoys, or dislikes, something, you shouldn't forget that anyone disagreeing with you isn't necessarily wrong, our out to get you. Trying to see it the way other people might see is a lesson I'd like a lot of people to learn.
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A lot of people like cocaine. A lot of 'trashy' movies have cult followings. Hell, people know some things are bad (for them) and enjoy them anyway, for the sake of them being bad.
There is no such thing as an ultimate 'good' or 'bad' for any one piece of art. That doesn't mean it is beyond criticism. There are things you can rank, or describe as bad or good, in most things. And you can find good and bad both in most things, as well.
Take Rings of Power, for example. For me, it's pretty bad. But that scene with Elrond meeting Durin the 4th again, after 20 years? I thought that was pretty good. It perfectly showed how different the two species and their perception of time was. There were some actual emotions in there.
I generally think people should 'live and let live' when it comes to entertainment. As long as people keep to themselves, whether they like it or dislike something, I'm fine with it. If you go off on someone because he hates/likes a show you hate/like, that's when the issues start to pile up.
And never mislable your opinion as fact.