I use Google Drive for main documents due to its reliability (it's Google, the datacenter king... what can go wrong?).
Dropbox for easy sync tasks, and MEGA for simple/temporary file storage (I am aware of MEGA's notoriety lol).
Another cloud service (for temporary file/screenshot/code snippet/etc sharing) that I use is
puush, via the ShareX client.
Do keep in mind that there are unlimited bandwidth/storage space services like Amazon and Backblaze for storing literally all of your files (not recommended to rely 100% on them of course, multiple backups are always recommended).
Another feature I'd look for in cloud software is automatic file versioning/history (although I wouldn't use it
).