In addition to redundant copies on USB drives stored at different locations, I suggest burning the entire source tree to CD/DVD and placing it in a safe deposit box with a bank. Of course, you'd need to 'refresh' that on a regular basis.

In some situations that is an acceptable solution to escrow requriements too. But you'd need to consult a lawyer on that.

IMHO any cloud solution is not secure enough for truly valuable software. Of course, you could encrypt and use more than one service - e.g. DropBox and iCloud and GoogleDrive. But ...

Burn CDs, make USB copies, and print out source code (on paper).

I would steer clear of internet-based services.

If it's worth million dollars, you can pay a security expert to back it up properly.

bitbucket.org - keep it in free private repository. Make occasional backups and send it to myself as encrypted email attachments.