I think Safest place will be Hardware wallet but sometimes I cant store all Alt-coins in hardware wallet. Below are few things which you must take care while buy, selling or storing your cryptocurrency:
1. Securing a Wallet is similar to securing your personal data on the computer. You have to be aware of links, advertisement, any type of attachment etc. which appears when a browser is being suffered.
2. Always create backups of your cryptocurrency wallets. So, that it will let you restore your wallet if in case your laptop or cell phone gets stolen.