Hmm looking at my OS usage history, here goes:
MS-DOS---------------------
I didn't really use it back in the day that much aside from very light usage. I mean I was very young so I mostly used it to boot up games and asked my older brother to (happily) do all the configuration stuff for me

During my teenage years I had older machines that were running Win95/98SE and I occasionally booted them to DOS to play some games that weren't so happy to be run from inside Windows, but I was always limited to very basic understanding of DOS, mostly because I could do all the file organisation and stuff from inside Windows.
Windows 95---------------------
I skipped 3.1 and started straight at Windows 95. Now I liked it a lot but I don't have very much real nostalgia for it because I was so young that the memories are all quite fuzzy.
Windows 98 SE------------------------------
Now we're talking! I used Windows 98 SE extensively in my pre-teen and early teen years. I would say that this OS I have the 2nd most nostalgia for, and I'm planning to install it on one of my old machines at some point.
Windows XP----------------------
This is it for me. All of my teenage year memories reside here. All the wonderful video games, my first leaps into music production, the MSN Messenger, the relatively early days of web surfing and forum posting, it all happened on Windows XP. I used it till very late 00's. I still remember the day when my Win XP install got blasted with viruses and I had to wipe it! I accidentally had left my antivirus turned off, which I did routinely to save the precious CPU cycles when playing offline games, then I browsed some sketchy website or another and the whole system got pwned. The horror

Which then lead me to install...
Windows Vista--------------------------
I don't know what it is about Vista, but it never really 'clicked' for me. It felt like it was held together with duct tape and prayers. It was slow & sluggish, there were always some error or another, it was a pain to get old software to run and it always felt unfinished. Even though I was running it on hardware that was well above the recommended system specs. Looking back, to me it feels like an alpha/beta release of Windows 7, that sums it up pretty well I think.
It looked gorgeous though. I eventually moved on from Vista because I got stuck in an infinite boot loop during a failed Windows update

Nothing worked, not safe mode, nothing.
Windows 7------------------
I can't really say I can feel nostalgic for Windows 7 yet, because I use it currently as my main OS. I can say though that I feel wistful that it is only a matter of time before I have to truly move on and the OS will be shelved for mostly legacy offline specific-purpose machines. There is something about 7, that no Windows has managed to capture since. It has a soul. Just... look at the default Aero theme with the default wallpaper and it's easy to see... it's just so gorgeous. Everything just works. It's at the same time very utilitarian and minimalistic yet it is delightfully aesthetic. It's... it's just perfection. *Sniff* - here I am, getting all teary eyed over an computer operating system
So uhh, overall, I would say that the most nostalgic OS for me, as of now, is definitely Windows XP. It was with me throughout my awkward teenage years, and the memories are strong.

Edit: Typos