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Neither Dropbox nor SkyDrive can boast this unique feature of Google Docs. When you create a form, you can define fill-in-the-blanks fields, checkboxes, and options, and then publish the result as a survey, using a link or email.
When you open a form you sent out for others to fill in, you see the data in a spreadsheet, ready for tabulating or charting.
It's a very handy feature, and one you can't do with the other alternatives.