Bitrix24 has a number of advantages over SharePoint. First, it's essentially free. Second, all the tools that come with it - video calls, group chat, document management, CRM, file sharing, project management - are already integrated and work out of the box. You don't have to pay integrators thousands of dollars just to make SharePoint 'play nice' with other tools you are using. Finally, source code for Bitrix24 is available, so you can modify it as necessary. Here's side by side feature comparison that shows why Bitrix24 is the best free SharePoint alternative.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.