For me, the best linux distribution is still Ubuntu. I've been using Linux for more than 10 years, it's on my main PC, my work PC (when I can), and all my other laptops/computers (well, except one cheap chromebook).
I chose Ubuntu for my main PC in 2014 after playing with a couple distros and I've been using it ever since. At the time it was an obvious choice, but water has flowed under the bridges and a couple other good looking distros have popped-up !
I recently looked at all the good looking ones I could find and (sadly?) I've been disappointed. Most of them are in fact Ubuntu-based, that must mean something...
Why Ubuntu is still the best
Because it's a very polished experience
Every aspects of the OS and the desktop environment is polished to fit within the "Ubuntu experience". I rarely encounter visual bugs or design flaws.
Especially for a main PC (or an OS you show someone to convince them that Linux is the best), you need something solid, cohesive and that's what you get with Ubuntu.
Because it looks modern
So many linux distros look an feel old, not Ubuntu.
Because it's stable
Being a Debian-based distribution, packages are solid and stable.
Because it's well supported
The community behind Ubuntu is bigger and it's so much easier to find solutions online for problems you may encounter.
Because : perfect dark theme
I use dark themes wherever I can now, and Ubuntu's is very well balanced for use during the day and also during the night. It's not too dark, not too bright, colors are neutral. My eyes are very happy !
Because there is not much things to be unhappy about
Well, besides that time they included Amazon stuff by default, or the fact that they try very hard to push their Snaps forward...
The only thing I began to be unhappy about is the title bar, I think it's a waste of space considering you have a dock that mostly contains empty space (who completely fills his dock with applications shortcuts ? not me). But that can be fixed with gnome extensions anyway.
Other than that, no complaints !
Ubuntu is still the best option if you want a Linux-based OS that you can count on. I'll still be using it for the next couple years !
Other distributions I've considered that are worth a look :