Having used Snow Leopard at work, try leaving a .pdf file highlighted in Finder, then exporting a QXP file as that same .pdf. Finder crashes every time, corrupting the .pdf. I've lost count over how many times this happened to me, my blood pressure still hasn't recovered. Plus QXP 8.1 crashed all the goddamn time, it's easy to run things in Rosetta without noticing (I did!).
Plus there's little things like window snapping and resizing being much improved (dunno which release this happened), spotlight being better, etc. Searching in Finder seems to have regressed somehow though. Spaces is inferior to the later multi-desktop implementation imo.
El Cap is where I started, and it was good. Was it Mountain Lion or Lion that had the bug where it would eat up all the available RAM? I remember that bug hitting several school computers, making them slow because everything was constantly swapping. That was a headache.
Mojave has APFS, the dynamic desktop (which I think is cool!), and stacks, which surprisingly came in handy more than I thought.
I dunno, I'm a big Mojave fanboy. I don't think Snow Leopard aged that well, I tend to lump it in with mid-classic OSX (Panther, Leopard)