I prefer the modern UI, but I agree snow leopard was amazingNo. Snow Leopard was perfect.
I've found that catalina is more stable and fast than mojave, but El Capitan in my experience trumps bothI am on Mojave now and much prefer the look of dark mode over light. I remember when Yosemite came out with the dark menu bar and Dock (partial dark mode), I expected the next version would have a full dark mode. Instead it took four years and Apple chose to stop supporting many older Macs in the process. Very frustrating.
But I digress. Admittedly El Capitan was more stable and the performance was decent on an SSD. By contrast, I have FileVault turned on, and for some reason the Mojave 10.14.6 update lengthened my iMac's boot times considerably. I also get brief system freezes occasionally just trying to open the Notification Center.