Windows 7 is my all-time favorite, but I personally don't consider it /retro/ because I used it from 2010 to 2020, so I'll have to say Windows XP. I used XP daily from around 2002 to 2007, and it was the OS where I learned most of my foundational knowledge regarding computers, software, internet, etc. Earlier this year I even dug out and upgraded an old Pentium 4-based XP machine I've had lying around for years just for fun/nostalgia purposes. It surprised me how usable it still is, at least provided I give it enough RAM and go back to being very conservative with multi-tasking. If push came to shove, I could probably still use it as my main OS, which is a comforting thought.
I have fond memories of Windows 95 too, but we didn't have internet back then and I was too young at that time to really do anything besides play games and click on random shortcuts, so I didn't get the most out of it like I did with XP.
I vaguely remember messing around with it as a kid, but I don't think I realized it was supposed to be an online chat thing due to not having internet. I remember being at my older cousin's house around 1997/1998 and watching him use some kind of instant messenger to talk to one of his friends, and I remember being confused as to how this was even possible. Even the fact they were using psuedonyms like "Alien" kinda freaked me out.