As far as "holy shit" improvements go - switching to 2k from 98se on my K6-2 350 was absolutely amazing.
Actually the single best moment in my computing history _ever_ was installing a Voodoo3 2000 PCI into that same machine (while under 98se), fighting to disable the onboard Rage PRO LT, and firing up Quake 3, Rogue Squadron, and Carmageddon 2 for the first time, and having silky OpenGL / GLIDE. That was amazingly cool.
As far as favorite OS's go, DOS 6.22 and Win 3.11 were my real halcyon days. I remember loading up boot disks to get Strike Commander and Wing Commander 2 running perfectly. TiE Fighter in DOS was probably the best thing ever.
I really liked my Macintosh IIvx, which had an integrated caddy-load CD-ROM drive. I got online for the first time with this machine, and did so until 1999 With System 7.5, it was a fantastic machine!!
Nowadays Vista 64 is the only OS for me. It's fast as shite, despite being massively bloated and having a few UI oddities (why can't I just have ALL explorer windows open in detail view, with the same attributes that XP displays? I don't care about star rating, and other media related metadata). It's also very good at managing video memory, and providing 3d acceleration to Adobe Lightroom, which XP cannot do!
~MiSfit