Either a paper wallet or a hardware wallet. Personally I use the Ledger Nano S which is great, I can store the private keys for multiple different coins (with new ones being added regularly) and access them securely with the device. There's also Trezor which is very similar, I haven't used that one though so can't comment on how good it is, I know it gets good reviews though.

Basically you need something secure and durable (if going down the hardware route), if going with a paper/offline wallet then somewhere secure to store your private keys!