Heh..yes, Second Reality is a classic. Anyone seen the C64 version? It's pretty good.
I know I've seen a couple of PC demos that I prefer, but right now Jizz by TBL is the one that comes to mind.
I'm more of an Amiga demo guy, however (I have a whole CD of them somewhere).. some of my favorites include:
State Of The Art (Spaceballs) - classic!
3D Demo ][ (Anarchy) - always liked the music and the starwars scroller is pretty cool.
242 (Virtual Dreams/Fairlight) - pretty cool for the time, features some neat video compression.
edit - there's also this one demo by Kefrens that I can't remember the name of, but it's pretty awesome, if only for the big cannon that emerges from one of the pyramids.
I'm also a huge fan of Da Jormas... I like just about all of their stuff, more for the music than for the technical part of the demos though, obviously.