My first Mac experience was some time in the late 90s with a Quadra running 7.5, so I'm super nostalgic for that. And somehow, both PB 1400s I have require 8.0, so I'll probably end up refurbishing one of my 5300cs laptops for that…
I do feel some amount of nostalgia for Mac OS 9, but I have a rather uninterrupted period of running it from 2008 to present so it's hard to quantify those feelings as nostalgia. It is more like a reliable friend I would never want to lose, but haven't lost, so have no reason to feel nostalgia.
On the X side… I have a really soft spot for Jaguar as it was the first X I used. My friend's Lamp G4 had it on. Sadly, she died a few years ago, but every time I boot up Jaguar on my iBook, I think of her and the stories we shared.
Tiger has some serious nostalgia as well, as not only is it the main OS I've ran on my G4 from the time I got it (it's still there today!), but it is also what got me into the Hackintosh scene when the Intel releases dropped. I was lucky enough to have the Intel reference board in my desktop PC that was used by Apple for some model of the original Intel Mac. At this point I don't remember which Mac, but I do remember the board was an Intel S5000XVN workstation board, so it was probably the MacPro1,1. It was quite easy to convince Tiger to run on it, and it was pretty funny / novel at the time to have the G4 and Intel computers side by side running the same OS.
Snow Leopard was the only OS I bought at retail (similar to CC_333 above me), so that has some nostalgia. It was nice to connect to Exchange in Mail instead of Entourage, I'll say that…
I do have fond memories of being an app developer during the Mountain Lion days, and that version of Xcode was hackable enough that I was able to ship a Really Universal Binary for 32-bit and 64-bit PowerPC and Intel using it. Huh, despite my hatred of Intels, I guess I did have some fun with them at times
I think at some point I will have nostalgia for Big Sur. I didn't have any Macs new enough to run it until the M1, so that's my first dip back into the Apple RISC Ecosystem. Right now I just remember all the "teething" issues so… not yet, haha.