Front + Intercom which gives us an email-based help desk, in-app chat and messaging, and a self-service FAQ help desk.
We're still using Zendesk for Help Centre, but considering alternatives. Last time I checked, Intercom's help centre product was still lacking some features we needed, unfortunately.
I think a big opportunity for Front is to make it easier to source content from different knowledge base systems (like Zendesk) to include snippets/links in emails without leaving Front. Perhaps a search box that inserts linked titles for Help Centre articles.