The best way currently, that is still easy to access are hardware wallets. The two most famous retailers are:

Trezor: https://buytrezor.com/
Ledger: https://www.ledgerwallet.com/

A hardware wallet is dedicated hardware with a defined interface that makes it impossible to extract the private key from the hardware. The interface does, however, allow signing of transactions. This means that the hardware wallet can safely be plugged into hardware infected with malware.