Ubuntu and Linux Mint are some good distributions to start with.
If you prefer more of a challenge, try Fedora, Debian, Arch Linux, or Gentoo (in increasing order of difficulty).
You should buy a new one that runs Ubuntu or Sailfish OS, or install one of them on your existing device if you can.
Since GNU/Linux is open-source, anyone can examine its code and fix or report any vulnerabilities they find. This means that security problems get fixed quicker than in other operating systems, making it more difficult for attackers to target GNU/Linux machines.
With GNU/Linux, software is installed and updated using a centralized package manager. Each distribution cryptographically signs their packages, making it almost impossible for an attacker to replace the software you use with malicious versions without getting caught.
GNU/Linux is free software, which means that it respects your freedoms to use, modify, and share it and anything you create using GNU/Linux. You are free to use GNU/Linux for any purpose, commercial or otherwise, without any restrictions.
Other operating systems usually do not allow you to modify or share them, or even to just look at their source code to make sure they're not doing anything nasty. Also, their vendors may restrict what you do with them, e.g. how many devices you can connect to your computer (Windows and OS X), whether you can run a server using your computer (Windows and OS X), whether you can use your computer for commercial purposes (OS X and some Windows licenses), whether you can connect to your computer remotely (Windows and OS X), or whether you can run the operating system on unapproved hardware (OS X).
Some operating system vendors, including Microsoft and Apple, do not allow you to use their operating systems to break the law. Now, this is perfectly fine when it comes to just laws (and you still have to follow them anyway), but you may also be subject to unjust laws, like laws prohibiting protests, acts of civil disobedience, or criticizing the government, or other censorship laws, that you should be able to break.
Some operating system vendors, like Microsoft, also require you to agree to binding arbitration in order to use their operating systems. This is bad for many reasons, but especially because it restricts your access to the courts and your ability to participate in class-action lawsuits.
Your computer should not be able to tell you what you can and cannot
do.
And if you're running GNU/Linux, it won't.