I really like Slack. We used HipChat earlier on, but had some friends who showed me Slack instead. It's so much sleeker and easier to navigate. It hardly cost us anything to make the change, so we switched over, and we've been very happy with it.
I love Help Scout! It's super simple and not nearly as complicated as Zendesk. Depending on the topic, the issue automatically gets assigned to different people, and it can even auto-respond. The only drawback to it is that it isn't in real time like some of the other software. You have to actively refresh the page to get the newest emails, which can be a big hassle.