Ubuntu has always been first choice for almost every beginner to Linux world. Its easy to install, with good GUI. It is based on Debian. It is composed mostly of free and open-source software. Ubuntu is officially released in three editions: Desktop, Server, and Core for Internet of things devices and robots. All the editions can run on the computer alone, or in a virtual machine. You can download it from Official Ubuntu Website.
I personally recommend this OS to everyone to try at least one time. It is super productive OS built on top of Ubuntu. It's Lite version is also available that runs smooth on older machines. I have ran them on machines having 1.5GB RAM and they worked fine. It is astonishing to see how software runs smoothly on the these older machines giving you modern experience. The Zorin Core OS also has phone connectivity feature through which you can control laptop through your smartphone, share the files from and to phone,can use smartphones keyboard to type on laptop also you can control mouse using smartphone's screen. You can download it from Official Zorin Website.
Elementory OS in simple words can be described as open-source alternative which gives you feel of using macOS. Elementary OS is a Linux distribution based on Ubuntu LTS. It promotes itself as a “fast, open, and privacy-respecting” replacement to macOS and Windows and has a pay-what-you-want model. The operating system, the desktop environment, and accompanying applications are developed and maintained by Elementary, Inc. You can download it from Official Elementory Website.
When you heard of Kali Linux most of you might remember the hacking scenes from movies but actually Kali Linux is a lot more useful distro to understand the the underlying working of an operating system. Even if you are not interested in Ethical hacking every programmer must try this distro at once to know more about networks and other related stuff.Kali Linux is a Debian-based Linux distribution geared towards various information security tasks, such as Penetration Testing, Security Research, Computer Forensics and Reverse Engineering. You can download it from Official Kali Linux Website.
MX Linux is a popular distro among programmers due to its lightning speed and Graphics. It's popular because it makes Debian more user friendly for beginning to intermediate (Not so much “non technical”) Linux users. It has newer packages from Debian backports repos; vanilla Debian uses older packages. MX users also benefit from custom tools which are great time savers. You can download it from Official MX Linux Website.