I use Box.com and subscribed when they were giving away an extra 25GB with a subscription.
That makes a total of 50GB for free. At least for now.
It does everything I need.
I pretty much have all the above mentioned plus sugarsync. With them you get 5 GB free but lots of ways to gain space, so I am up to 8.9 GB now for free. Plus the app will back-up videos and pics if you want. Want an account sign up here and we both get bonus storage:
The thing I like about DropBox is the seamless photo synch from phone to computer.
I take a pic on Dinc2. I turn on computer. BAM. Pic arrives.
It's super sweet.
They keep upping the free account limit too. Used to be 10. Then 12. Then 16. Now 18.
I also have Box (50GB) but once I started using the photo synch thing... Dropbox all the way.
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.
I've used Dropbox Free for years and over that time slowly got it up to around 8gigs of free space from referrals. Then after getting my S3 and putting a ROM on it, I got the 50 gigs free for 2 years. That is crazy good for me. I do pay for 20 (or is it 25) gigs of space from Google Drive. My primary one is Dropbox though.