r/changemyview is a good one, it's people in conversation about contentious topics, politely trying to change each other's views. There's very little censorship of discussion compared to the rest of Reddit, the only rule seems to be politeness and topicality. It's almost like HN in that sense.
Very much disagree on this point - I've both started threads there and contributed to threads there in response, and twice admins have locked my thread beacuse they claim I had no intention to change my mind in the first place - as far as I can tell, if you debate people trying to change your view, then you're soapboxing, rather than trying to clarify the argument.
I haven't posted on r/changemyview since. It's not fit for purpose, and in fact many threads consist of the OP saying "Wow, you're right! Mind changed." to a rather poor argument rather than interrogating it.