1. Dropbox for all the read-only ( Images, PDFs, Books, important papers).
2. I use IFTTT to autobackup a specific folder (very imp one) from Dropbox to Box.
3. Something I need to edit, like spreadship or docs. GoogleDrive works very well.
4. I keep most important files on atleast two services.
5. IFTTT works great to collect data automaticaly.
Edit: Goodlooking line breaks inserted.