Boxcryptor secures your files with an encryption program made in Germany. It has a free level and upgraded levels that cost a monthly fee based on your needs. Only you can access your files when you have Boxcryptor and it supports nearly every cloud provider.
Boxcryptor is an incredible free option to protect your cloud data with practically no downside. If your concern is cloud security then Boxcryptor is the best option.
Boxcryptor allows you to protect your data across all platforms without anyone but you being able to access it and it is free.