Onedrive, but used to use Dropbox.
Dropbox is way too stingy with their storage and their privacy policy is utter shit. Google Drive is fine but also has privacy issues. MEGA gives you a ton of storage but sync app can be wonky (changes file name of uploaded pics from a phone to date uploaded, not date taken).
Onedrive gives 15GB for free, 15 more if you enable photo upload, a terabyte if you have Office 365, and the sync works wonderfully. They even have less privacy issues because here's the thing: Microsoft doesn't care about the consumer market, they only care about enterprise. If a business or employees store work docs on Onedrive, they HAVE to know it's secure. Microsoft can touch it, but they don't want to as it would hurt business with these corporations. Only downside is no Linux sync application (Onedrive-d doesn't work well).
