On OCS/ECS: ARTE/Sanity, Desert Dream/Kefrens, 9Fingers & State of the Art/Spaceballs, Hardwired. Most of the Andromeda stuff (Sequential, D.O.S., Mirror etc.). Some Lego intro's were quite cool.
On AGA: Nexus 7/Andromeda, Relic/Nerve Axis (I wonder why nobody suggested this one so far), MegaDemoIV/Artwork, most of the TBL stuff (Glow, Tint, Goa, Captured Dreams, Rain, Perfect Circle), Hotstyle Takeover/Loonies (for running in one frame), The Law & Repugnance from Powerline, most of the Mellow Chips demos, Motion/Bomb.
4k Intros: Exploder & Azure stuff.
The most impressing PPC 64k intro so far: Planet Potion by Potion.