As most people here have stated, I use a mixture. Dropbox has been my go to since... 2011 I believe but I'm trying to switch over to using OneDrive more due to the massive amount of space available. The only problem is the IT department at my work is, ahem, somewhat behind the times and might very well give me a blank stare if I talk to them about OneDrive, whereas they've at least heard of Dropbox.
iCloud Drive has been crippled for years because it doesn't play nice with anything? Only recently (since moving fully to Mac, office computer excepted) have I been able to get full access to iCloud as a folder similar to Dropbox, but it's space is limited and I dunno why in the world I'd use it for anything other than syncing contacts and backups.