I have three that I use in my classroom; the one on my main desk is a 21" 2010 that handles everything I throw at it beautifully, and a 20" 2008 in my office that handles the same tasks, just somewhat slower
Both of these machines work wonderfully, and the 2010 in particular is relied on to do everything I need to run my classroom effectively. I do much of my lesson planning and grading on this computer. I teach Robotics, so I end up running a fair bit of basic 3D modeling software on it, along with several flavors of programming software. Both of these machines serve me well, and they were both super cheap!
I have the RAM in both of them maxed out at 16GB and 6GB respectively, and both are outfitted with SSDs. With these upgrades, they run beautifully. The 2008 runs High Sierra courtesy of @dosdude1 and his patcher.
The third one is a 20" late 2006 iMac running Lion that I use as a message board computer to put various messages and information for the students. It's run with no keyboard or mouse; I remote control it from my main machine. It sits on a table by the door where the students pick things up, and has performed its basic job well for several years now!
The range is from 2002 to 2004. It looks like a 15" though.
You sort of beat me to it. Here is the specs of my 2002 iMac, 17"
I did have a 2015 27" 5K iMac, but I sold it not long ago. So my vintage G4 iMac is the only one I have. And yes, I do use it. It is just not connected to the internet. But it can still run some great old school apps.