I think that it is hard to beat r/ChangeMyView (https://www.reddit.com/r/changemyview/), as the arguments are usually much more detailed than "left vs right".

However, there is one crucial prerequisite - someone needs to have an open mind, and courage to at least doubt one's own point of view. Compare and contrast with Change my View by Steven Crowder, which is full of fake openness + trolling, not genuine curiosity.