The best platform ?
If you are serious about innovation, it's the wrong question to be asking. Innovation does not start with the platform, its starts with the people you want to bring together. Who are they? What are their personal characteristics? What do they think innovation is? What are the themes or topics you, and they, think relate to your innovation situation? Then align what you learn to the "characteristics" of a customized (not prefabbed) environment for collaborating. You will find that the collaborative design environment (not platform) will evolve over time, as your interactions reflect nuances of need that can only come through experiencial engagement. Allow the making of your collaborative design environment to be a part of your innovation process, rather than something separate from it.
My thoughts, Vic D.