For me it would have to be around the Quake/Quake 2 timeframe. It really went ballistic for me when Half-Life came out
However, there is a vast difference between the two platforms, since I'd have to say that really the Demos/Megademos on the Amiga just plain kicked any machines ass, past, present and still into the future. There are some great PC demos to be sure but they, for some reason, don't have the same look and sound quality that the Amiga gives you.
Again, I strayed from the specific topic a bit, but I think in a way, demos are really a peak into the technology that powers them, this is why the Amiga is so far ahead of it's time.
Guess I'm getting a bad cause of nastalgia here, so I'll leave it at that