For long term/high number of coins (example: 100 BTC) - anything that is not physically connected to a PC. Be it a piece of paper, engraved metal plates, manually carving it in wood, etc.
- For medium-sized coin piles, used for transfers from time to time - the native wallet of the coin.
- For daily use with small amounts - the most convenient, easy to use a mobile wallet.
*high/medium/small is relative, varying from person to person.