Sharepoint locks you into a box, but at least most MS Office products are in that box and work well together. Sharepoint tended to be very "clunky" when used, but it covers the basics of collaboration even if it could be difficult to use. I can't speak from an Admin perspective, but from a user side it fulfilled its purpose.
Salesforce Communities/Chatter - I include this because if everyone is onboard with using Salesforce, Chatter and Communities can be a solid platform choice for collaboration. Usually business data is in Salesforce already, so having conversations and collaboration around the specific business information is extremely helpful. Being an administrator, I viewed the administration of Salesforce to be complex at times, but quickly resolved.
My choice by order of preference would be: