1. Place Ss in groups or 3 and have then list what they know about the topic. They can do that either by rating (e.g., rate the best movie/song/book), by ranking (e.g., rank the best movie/song/book) or by listing what they know about a given movie/song/book.
2. Elicit the groups' findings, and while doing so make sure to note down on the board the words and phrases they are using to describe their findings. Make sure to correct the errors and to add in new words and phrases for them to learn. What that does is prepare them for the kind of language they are going to need for the discussion proper.
3. Have 1 person from each group switch with another group. First, have the members share their findings with the new person. Likewise, have the new member share his/her ideas with the new group. Then, modify the topic (e.g., why do you think this movie/song/book is the best?), but before they can discuss that, they'll need a few phrases to work with (e.g., I think/feel/believe this movie is great because/for several reasons/, etc.; This movie has, is, etc.; and so on. You don't have to come up with everything here; ask your students to add to what you have already given them. Find out what they already know and add to it for them. (Doing that extends their present language base. Not doing that results in class discussions that go no where fast.) Now have the groups discuss the "Why" question.
4. Elicit the groups' findings (e.g., what did X have to say?) While doing so, (i) write down the language they're using, (ii) correct it, and (iii) add to it.
5. Modify the topic question again. Have the students stand and mingle this time. It's important to get them up and moving around as it's not as easy to think on your feet as it is sitting comfortably hidden safely in a chair behind a desk.
6. Elicit their findings. Write, correct, and add new language.
7. Now they are ready to discuss the topic, which doesn't and shouldn't have anything to do with movies/songs/books they think are the best. The point of the lesson is to provide them with language to talk about, say, "Giving one's opinion/Disagreeing/Agreeing, and so on.
Following the procedure above will not only have your students invested in the topic, just as importantly, it will give them the language they will need to express their ideas and opinions in a way that makes them feel empowered.