With so many subtle differences it’s hard to say best with these services but I ended up using box.com simply because others had sent me invitations to share holiday pictures and such. What I found afterwards is that most of my ipad apps support box.com as a means for storing files. I think Dropbox is supported at least as well.
I have a little more than 2 Gb stored so it’s still free on box, also I like the ability to edit some of my docs online through box.
It may just depend on the apps you use most frequently and which services they support.