My favourite (oldskool) demos are Alcatraz: Odyssey (the 40 minutes long show), Phenomena: Enigma (esp. the music of Tip), Budbrain: Megademo (the toilet :-D ) and Spaceballs: State of the Art.
These demos are available in WHDLoad version too. The "nowadays" demos differ, that it's gfx mostly drawn at pc (then converted), the sound is open mp3 (then downsampled). Some of them requires 3D card (Potion intros, MAWI demos), which I would like to watch, but I haven't got a 3D card (because only few Amiga programs use it).
The old demos had much better atmosphere. To learning code on Amiga was very hard (programming books hard to come by), almost all of the graphic was pixelized (just some digitized, because that time appeared digitizers) and great sounds delivered (Tip,Mantronix,Firefox).
bye,
enzo