Which OS is the most secure when I want to use a crypto-wallet and make transactions? I heard that Ubuntu, is it right? 
ubuntu? No way.
Ubuntu is one of User friendly distributions have a good community and good support behind it but not known as safe or privacy focus distro at all.
It had many security problems during years.
Even there was scandals for Ubuntu . last one i know was "Ubuntu Spyware".
Mint could be a good choice for anyone new to this area to learn what gnu/linux is.
Among the security focus distros the best choice could be tails as NeuroticFish mentioned above. has electrum wallet by default.
You can read more here.(https://tails.boum.org/) its just live distro and not recomended for newcomers.
And check all distros popularity here. (https://distrowatch.com)
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Offcourse, an end user can make the best designed OS vulnerable, or the worst designed OS secure
Its not true. you could not,Can not, and will not be able to Secure windows Ever.
I'm not going to start a pointless discussion, but I wholeheartly believe there is no 100% secure os. Even the os you recommended has had vulnerabilitys at some point.
As you might have noticed from the part of my post you didn't quote, I'm not a Microsoft fanboy either. As a matter of fact, I maintain Linux servers as my daytime job. I still stand by my earlyer statement... For example: I'd rather run a tool on a windows server that has been maintained by a skilled windows sysadmin that had enough time and funding to do and maintain a proper setup compared to running a binary on a Linux box that has been installed and used by a newbie that made a lot of mistakes (like running everything as root, installing unknown binaries, not updating, no firewall,...)