Hey there :)
As per question, the best way is Encryption.
You may feel this as wrong answer but as per the latest scrutiny, I'll explain:-
Option: Antivirus (NO)
This is safe and does the job to some extent. But none has ever found with clear proof of the best antivirus. Everyone of those antivruses has some pros and cons.
Some of them does the job of protection but with them, they also give us some threats !
Option: Firewall (No)
Firewall also helps to catch and delete malicious softwares to 79%, but it doesnt give you the maximum security from data breaches.
Option: Password (Obviously NO)
Passwords may help in personal accounts and bank, but they are useless in online world. Nothing can be protected fully by passwords. Also, password is the easiest way for cyber criminals to attack.
So, answer is "Encryption"