Zulip has easily the best threading model among all the chat applications and I prefer it over Slack, Mattermost, RocketChat, Hipchat, Discord etc. Each and every conversation is a seperate thread and has a topic. This model makes it extremely easier to catch up and participate in conversations. Once you get used to the threading model of Zulip its hard to tolerate threading model like Slack which is really inefficient and time wasting.