If you are concerned with durability mostly in terms of the next few decades, I think S3 deep archive would be difficult to beat. Your data is offline by default. It would be logistically infeasible for a rogue agent to physically mount every tape for purging before someone would notice and stop the tape library.
The risk with any cloud is that while your data might be physically intact, you can still lose access to it for a variety of reasons (e.g. being born in a wrong country)
