Many have already mentioned that the most likely safest solution for long-term Bitcoin storage typically involves using a hardware wallet. However, I would like to add that if it is truly for the long term, in that case, you may not even need a wallet. Keeping your seed phrase in a safe and secure location (preferably in multiple copies) could suffice. Just make sure to generate the wallet on a clean and air-gapped device, write down the seed phrase (or even engrave it on a metal plate), export the public addresses that you will use for deposits, and then erase all digital traces of the wallet. Optionally, you can export xPub (or zPub) keys to external media to create a watch-only wallet on an online connected device if needed.