What can I do if I observe that a wrong and misleading answer is the most upvoted one among all the answers to a question and commenting, downvoting, and writing a better answer does not help and the OP of the wrong answer insists in being right?
Comment on it, and downvote. Post it in chat/meta if you feel it needs more scrutiny/discussion. DO NOT flag, flags aren't for wrong posts. Also, put a comment on the main question, asking the OP not to accept it.
If it's already accepted, ask the OP to unaccept. But you can't do much more than that. This has come up multiple times on the mother meta.
When you see that an answer is wrong, you can downvote and leave a comment explaining why it's wrong, and of course write your own answer (not as a response to the other answer). But that's pretty much it. In particular, if you go to chat or meta and ask people to downvote it outright, that's not really appropriate. You can use the chat room to discuss it with other people, and explain your reasoning for why you think it's wrong, but don't send messages like "This answer is wrong, you should downvote it." You're not forbidden to tell people how they should vote, but doing so is rather rude.
Asking the OP to unaccept the answer is likewise inappropriate, although it's sort of a gray area depending on how you phrase it. For example, if you post a comment saying "Please unaccept this answer, it's wrong," that's not okay, but if you say "I don't think this should be the accepted answer because ," that would probably be okay.
This very response will probably get me downvoted, but I don't care. I can say that after being shouted down for posting patently correct answers/explanations and essentially being accused of incompetence, I have decided to stop pointing out incorrect answers when I see them. The shouting down usually comes from one of two sources: grad students or researchers, neither of whom really take the time to think through better explanations to even the simplest of questions. There is far less subjectivity in explaining some things than we all want to believe or admit, and getting that point across is a struggle. Word choice matters because words influence thoughts, and that's where misconceptions can arise. I sometimes use anonymous examples from this site in class to illustrate such cases.
Ultimately, crowdsourcing correct answers by popular vote results in sites like this becoming unreliable resources for students (nevermind that the site may not have been conceived as being a reliable resource for students...we all know students at all levels come here). Although I'll stick around and use the site as a resource, I can never recommend it to even my best students.