Which OS is the most secure when I want to use a crypto-wallet and make transactions? I heard that Ubuntu, is it right? 
The most secure OS will be the one that you understand better.
For example, if you are using linux and some hacker get close to you computer, he could easy vuln it with:
Code:
service sshd start; firewall-cmd --permanent --zone=public --add-port=22/tcp
He could edit ssh to allow anonymous connections and your linux will be the most vuln system.
So, there are no secure OS, use Windows use Mac or use Linux, and end you need to know your system to be on the safe zone.