I guess it depends.
If you break it down a bit:
OCS/ECS game: A lot of candidates. Agony is gorgeous, the original Shadow of the Beast was something never seen before on home computers.
The Settlers, if you factor in how many little guy's the game is animating and keeping track of.
AGA: Banshee, maybe? Might be more technically impressive games, but it's got loads of detail, lots of colour and it's well drawn and animated
"Big" titles (CD rom etc): Onescapee or Genetic Species ? Both are stunning considering the hardware they can run on.