I see it's already been mentioned, but allow me to second Backblaze. I use it for hundreds of clients, and they have consistently been the most reliable of the

I see it's already been mentioned, but allow me to second Backblaze. I use it for hundreds of clients, and they have consistently been the most reliable of the backup services I have tested*. Since they also have versioning, you can recover from CryptoLocker variants comparatively easily as well. (Almost any cloud-based backup system can help detect Locker variants. If you notice your daily backup data set suddenly shooting up in size, time to start checking for background encryption.) (°Backblaze, Carbonite, Crashplan, Mozy, Acronis)