IRC is better than both. Slack would be ok if not for the lack of logging and the disappearance of text after a few days and discords server and channel security is just meh. Forums are better for many things that chat programs do poorly or not at all.
Forums are my favorite because it has a different feel, I like how everything stays and it feels a little more formal but slack isn't too far behind I just don't like how things disappear. (I'm sure that sounds like gibberish)