I use Google Drive, mainly because I used Google Docs a lot and I am fully entrenched in the Google universe (loving it).
I have accounts with Box, Dropbox, Skydrive, etc...and they seem nice enough, but for me, since I use many of Google's services, it's easier and more convenient to use Drive...which I like quite a bit anyway.
I would imagine if you are a new die-hard Outlook user, then Skydrive might be a better option since they interact somewhat.
Box gives me the most space...but I don't need it and it doens't merge with a lot of other stuff I am using, like Drive does.
Honestly, I'd rather pay for extra space on Google Drive when the time comes where I need it. For $2.50 a month, I can get another 25gb that is shared across Drive and Picasa...which is a great deal for those who use both often, as I do. PLUS, if you buy a storage plan, your Gmail is automatically upgraded to 25gb of storage.
Also, with Google Drive:
- Google Docs and files converted to Google Docs format don't take up storage space...and I convert everything I can anyway...so it will take me a while to use up my 5gb.
- On Picasa, files 2048x2048 or smaller don't count towards the 1gb limit. This limit applies to photos uploaded to Picasa Web Albums and photos uploaded at original size to Google+ from the Picasa 3.9 software...but by default, photos uploaded to Google+ get sized down to 2048 and don't count towards the limit. I have over 60 web albums that contain hundreds if not thousands of photos...and I am using 148mb of my 1gb limit.
So really, you get a LOT more storage space than the limits say if you convert your docs to Google Docs and let Picasa downsize your images to 2048x2048... infinite space really.