The best knowledge base software I've found to date has been Helpjuice. Helpjuice is very focused on 1 item, and that's being a great knowledge base. As a result, their product and service is far surperior to competition.

Some important highlights / why I like Helpjuice:

• Support for both on premise aswell as cloud hosted
• Amazing customization. Yes, this means they'll fully customize your knowledge base
• Powerful analytics around what your team is doing, and many more knowledge base features