Pinball Illusions was the first AGA title to blow my socks off. Excellent design, clever masking tricks, and one of the few [good] pinball games to use real blitter objects instead of bitplane shifting. People don't give this game the credit it deserves because there's not much moving. :-)
For OCS, I was fond of Battle Squadron, Project X, and Sword of Sodan (although the animation was crap and the blood spatters were truly laughable).
I think Superfrog has pretty good graphics
Smooth, but not very creative.
I've heard that some adventure game (Universe, IIRC) had 256 colors on ECS Amigas!!
Shadow of the Beast boasted 128 colors on the box. We all know that's the good ol' copper at work, though.
Mine is still Agony from the last time this came up
Nicely detailed, but bad use of color. Level 1 was basicly brown and green. Ugh. Also, too dithered. The loading screens are phenominal, though.