The fifth most valuable? Only the fifth?

We use Dropbox for teams for our company data. We forked out for it because it is that good. Admittedly we're small - only five people thus far - but growing. A few years ago we couldn't do this reliable without a file server. Dropbox has revolutionised file sharing from the low end, and will continue to hit up into the business market with their current trajectory.

Yes, we have used alternatives, but none are as straightforward and fuss-free as Dropbox. I think they're the most valuable startup out there at the moment.