The most controversial opinion I have is hard to say.
My most controversial is probably that I don’t believe there is anything wrong with incestuous relationships between consenting adults that don’t produce children. If it doesn’t harm a person, an animal, or the environment, why should I care what two adults do sexually/romantically? Who gives a crap if they’re related?
I don’t believe lying is always wrong and I don’t believe revenge is always wrong.
Ludwig van Beethoven is overrated. Doesn’t mean he isn’t good, but he’s not as good as people say.
There is a difference between a “pedophile” and a “child molester”. A child molester is a pedophile who acts on their desires. Pedophiles can simply never act on their desires but still be attracted to children. That doesn’t make them a child molester.
Offensive terms, like racist and sexist terms are not always offensive. It depends on the context of their usage, who’s using them, and for what purpose.
The most important one of all to me: “It’s gross”, “it’s different”, and “it’s uncommon” are NOT reasons for things to be considered wrong.