What is the absolutely safest but yet convenient way to store Nano long term?

Using web, desktop, mobile, hardware wallet or offline pc, paper wallet or multi-sig?

I would say purchasing a Ledger Nano device and put the recovery phrase on a paper. Cut the paper in half and store the two pieces in two safe locations (like safe deposit boxes) as far away apart as possible. Even better make a copy and store in 4 places for redundancy. The 8 digit pin you save in your brain. Hide the ledger inside a wall or otherwise easily accessible yet hard to find place. Write a will of where the papers are and what to do in case of death. Original Ledger not needed as long as the recovery phrase is intact.

That would protect against theft, fire, natural disaster, memory loss but you can still access the coins whenever you like without compromising the private key (as you would with a paper wallet). Another option for not compromising the private key is to use offline signing from a safe disconnected computer but that is way more complicated and easy to use tools are not yet available.

For more about the security vs. convenience topic, please have a look at this article: https://medium.com/nano-education/how-to-secure-your-cryptocurrency-nano-7a83b194e474