I'm using Mavericks on my main, every day computer, my late 2011 Macbook Pro.
With that said, I still use a couple of older computers on a regular basis, and as such can not upgrade them any further. My Powermac G5 has Leopard(10.5) on it and my "Quicksilver" G4(which probably gets used as much as the G5) is currently running Tiger(10.4) and OS 9. My G4 is capable of running Leopard, or at least has been since I upgraded it to dual 1ghz processors, but for the time being I'm sticking with Tiger.
I have another G4 lying around with multiple hard drives and partitions, and have 10.1, 10.3, and 10.4 installed on it(as well as OS 9). I have no particular reason for this, other than it's interesting to see the evolution of the operating system through the various versions.
Finally, I have a G3 that is running 10.2, the maximum version this particular computer is officially capable of running(although there are work-arounds that I haven't yet explored).
I started with Lion, and used it fairly extensively. I don't feel like the differences between Lion and Mavericks are that great(I skipped Mountain Lion), but I feel like many of the changes made in Mavericks are real improvements. In particular, when I'm using multiple monitors, the ability to switch between virtual desktops independently on the two monitors in Mavericks makes a big difference.