Yeap, my thoughts to..
OCS/ECS: Agony, Project X, Superfrog have impressive graphics while maintaining an ultra smooth frame rate. However, the list can be expanded to include several other titles from different genres as well, like MKII, Walker, Ruff 'n Tumble, SOTB series.. You know the drill here, just check the list of Psygnosis, Team17, Core et al. publishers
Still, I am visually in love with Flight of the Amazon Queen, since I believe that the main character is probably the best drawn/animated/controlable OCS human figure ever.
AmigaCD OnEscapee I believe has the most impressive artwork.
AGA.. hmm dunno I really like Aladdin but the fps rate really sucked compared to other ports of the game imho. Exodus has excellent background images (and imho is better than Napalm overal, but that's a different question
). Again here the list is rather bog.. for example Fightin Spirit was awesomely drawn as well!
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.