If you want the best all-around OS with the most user friendly GUI these days I’d at least start with Raspbian. I run this on my PiHole RPI and it works great. Since the Raspberry Pi folks “support” it or at least you can download it from their site, I’d say it is the default os. Some of the others are fun to try though. If you have more than one SD card it is pretty easy to do.
Some are close to real time, some are entertainment oriented, some are IOT oriented (Windows IoT in particular with Win10) and some are there because the Linux port is pretty easy to do and a lot of people have their favorites. There was even a Unix BSD port although I don’t know if it still exists.
Everybody seems to be using Python to add their own applications to the RPi and even the Arduino now so if you want to really get into the guts of the PI, Python is a great way to go.
Ultimately, I’d just experiment with it and see what works for you. Frankly, one of the best things about the little Pis are their cost. I can’t really tell you how many I have, but I’m not afraid to go get the latest.
I own three of the RPi4s, 2 2GB and one 4GB. You can have all kinds of fun for not a lot of money.
I don’t know if you’re a programmer or not, a lot of DMS folks are, but the RPis lend themselves to doing some pretty amazing things and GiTHub is packed with people’s projects you can try.