Dropbox is in my experience the easiest to use; it is easy to upload and share documents across mobile devices. That being said it is more consumer focused than business oriented, although they are making efforts to beef up their business offering. Box.com is more enterprise oriented - you can easily set up data rooms among internal and external teams of users; however I find Box's usability across different devices (laptops, iphone/pad, etc.) to be more difficult and cumbersome. My company is currently beta testing enterprise box. Both dropbox and box are easily scalable based on number of users. My understanding from my IT department is that Skydrive/OneDrive does not have the level of encryption and security that box and dropbox (and other cloud services) have.

Of course, but it doesn't do much in the way of versioning or collaboration on non-google apps style documents.

What services offer those features?

Neither can guarantee EU data compliance.

If OP really only wants storage then AWS or similar might serve just as well as anything. (Cue jokes about EU data compliance)

I remember seeing an article about a cloud service called Microsoft Azure, is it the same as office365?

No. Azure is an infrastructure-as-a-service offering, for companies to deploy software onto; it doesn't offer the kind of specific business functionality that Office 365 (or Google Apps for Business) does.

Box offers versioning control and web based commenting. Plus a user can easily view what changes have been made by other users to a document that was worked on locally and then synced to box via the desktop client (similar style to Dropbox / Google Drive). It's pretty cool.