Encrypting locally before uploading to the cloud is probably the best solution. That way unencrypted data never leaves your local device.

You can do this with an app like VeraCrypt https://veracrypt.codeplex.com for DropBox or SyncDocs https://syncdocs.com for Google Drive.

Relying on cloud providers to protect your data is not a great idea - just ask Hillary Clinton about Podesta. Adding a separate layer of security and another means of authentication is more secure.