#1: Shadow of the Beast - Goosebump inducing intro music and graphics. Still like to watch it through today. Shame the game itself was lacking in solid gameplay, but those parallax layers looked stunning fourteen years ago.
#2: Super Stardust, tunnel warp - More goosebumps and adrenaline, fantastic sequence.
#3: Elite 2: Frontier - Okay, so the intro ran rather sluggish on a vanilla A500/A1200, but nonetheless it was one hell of a demonstration for a freeform environment, ten years ago. Coincidentally my favourite game of all time.
#4: Pinball Dreams - Technically there was nothing impressive about this, but the intro music and graphics before choosing the table was just slick. No other word for it.
There's probably loads more, but my memory fails me at this time