Hard to say what is best without knowing what your requirements are. However, I use Amazon S3 99% of the time for anything I don't store on a local server. There are apps out there that can allow you to mount as a shared drive. If you are a Mac/Linux user you can use fuse s3fs to mount as a filesystem.
I've been using Copy because they give a lot of free space, but not for many other reasons. Dropbox remains king - especially if you consider editing. The new integration with Office is very nice.
what exactly does "best" mean here?<p>I've been using Dropbox since, well... a while, and I really appreciate the cross-platform compatibility (Linux/OS X)