The leading players are Zendesk, GrooveHQ, and FreshDesk. I have used all of these and my vote goes to Zendesk.

I talked about it as a case study for a documentation task, see http://www.vinishgarg.com/2014/09/kb-case-study-pain-points-and-analysis/. I am also co-founder of @In23Hours where we develop product KBs in Zendesk.

If you have any specific questions or doubts, setup a call and I can explain everything that you may need to know.