My first distro and one that will always have a special place. If it were not for Ubuntu in 2009, I daresay I would not have stayed with linux and would now still be on Windows. Very user friendly and if you stick to the LTS very stable.
Pop OS is a wonderful user friendly distro that would serve any new user well. It comes with extraordinary levels of polish and a great set of applications by default. Based on Ubuntu you get the stable base and System76 have done a lot of work to make the experience even better. As a gamer I can say that Pop OS is one of the top 2 distros for gaming experience that I have used (the other being Solus).
Solus is another fabulous distro that works well with minimum fuss. If you want a rolling distro that is rock solid, has a sane set of defaults and just stays out of your way, then Solus is one of the best. Yes, you may struggle if you have very niche application needs due to the curated repositories, but all the popular applications are present and accounted for so most users should be able to get along just fine. There are also snaps, flatpaks and appimages to plug a lot of gaps so application availability isn’t as big an issue as it used to be.
Xubuntu is a distro that I started using when the Unity desktop initially came to be and was not yet mature. I ended up loving Xubuntu and Xfce, and sticking with it for several years because it is an OS that you can always come back to when you break something else and need to get up and running quickly. It is a reliable work horse of an OS that will never let you down. It gets criticised for a glacial pace of development but for me that is its biggest strength.
I started with linux in 2009 back in the days of the Gnome 2 desktop and if it wasn’t for the simple intuitive experience I may have bounced of it. The Mate desktop is a beautiful throwback to that paradigm and for this reason I like to check in on it from time to time with Ubuntu Mate which, in my opinion, offers the best Mate experience. This is the distro that I will typically recommend to new users because they have several tools that make switching to linux easy, from the excellent welcome screen to the software boutique and the Mate tweak tool which offers a user friendly way to customise certain aspects of the desktop. It is also, like Xubuntu, legendarily reliable at this point which doesn’t hurt.